-
Notifications
You must be signed in to change notification settings - Fork 708
/
Copy pathOverview.bs
1162 lines (998 loc) · 44.3 KB
/
Overview.bs
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<pre class="metadata">
Title: CSS Overflow Module Level 5
Status: ED
Work Status: Revising
ED: https://drafts.csswg.org/css-overflow-5/
Shortname: css-overflow
Group: csswg
Level: 5
Editor: Florian Rivoal, On behalf of Bloomberg, http://florian.rivoal.net/, w3cid 43241
Editor: Elika J. Etemad / fantasai, Apple, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Robert Flack, Google, flackr@google.com, w3cid 98451
Abstract: This module contains the features of CSS relating to scrollable overflow handling in visual media.
It builds on the <a href="https://www.w3.org/TR/css-overflow-4/">CSS Overflow Module Level 4</a>,
adding an appendix containing an experimental exploration
of <a href="#fragmentation">redirecting overflow by fragmentation</a>.
</pre>
<pre class="link-defaults">
type: dfn; spec:css-multicol-1; text:overflow column
spec:css-pseudo-4; type:selector; text:::first-letter
spec:css-pseudo-4; type:selector; text:::first-line
spec:css-writing-modes-4; type:dfn; text:start
spec:css-writing-modes-4; type:dfn; text:end
</pre>
<pre class="anchors">
url: https://drafts.csswg.org/selectors-3/#subject; type: dfn; text: subject;
url: https://www.w3.org/TR/CSS21/visudet.html#strut; type: dfn; text: strut;
</pre>
<style>
table.source-demo-pair {
width: 100%;
}
.in-cards-demo {
width: 13em;
height: 8em;
padding: 4px;
border: medium solid blue;
margin: 6px;
font: medium/1.3 Times New Roman, Times, serif;
white-space: nowrap;
}
.bouncy-columns-demo {
width: 6em;
height: 10em;
float: left;
margin: 1em;
font: medium/1.25 Times New Roman, Times, serif;
white-space: nowrap;
}
.bouncy-columns-demo.one {
background: aqua; color: black;
transform: rotate(-3deg);
}
.bouncy-columns-demo.two {
background: yellow; color: black;
transform: rotate(3deg);
}
.article-font-inherit-demo {
font: 1em/1.25 Times New Roman, Times, serif;
white-space: nowrap;
}
.article-font-inherit-demo.one {
width: 12em;
font-size: 1.5em;
margin-bottom: 1em;
height: 4em;
}
.article-font-inherit-demo.two {
width: 11em;
margin-left: 5em;
margin-right: 2em;
}
.dark-columns-demo {
width: 6em;
height: 10em;
float: left;
margin-right: 1em;
font: medium/1.25 Times New Roman, Times, serif;
white-space: nowrap;
}
.dark-columns-demo.one {
background: aqua; color: black;
}
.dark-columns-demo.one :link {
color: blue;
}
.dark-columns-demo.one :visited {
color: purple;
}
.dark-columns-demo.two {
background: navy; color: white;
}
.dark-columns-demo.two :link {
color: aqua;
}
.dark-columns-demo.two :visited {
color: fuchsia;
}
.article-max-lines-demo {
font: 1em/1.25 Times New Roman, Times, serif;
white-space: nowrap;
}
.article-max-lines-demo.one::first-letter {
font-size: 2em;
line-height: 0.9;
}
.article-max-lines-demo.one {
font-size: 1.5em;
width: 16em;
}
.article-max-lines-demo.two {
width: 11.5em;
float: left; margin-right: 1em;
}
.article-max-lines-demo.three {
width: 11.5em;
float: left;
}
.awesome-table td { padding: 5px; }
.awesome-table {
color: #000;
background: #fff;
margin: auto;
}
</style>
<h2 id="intro">
Introduction</h2>
This specification extends [[!CSS-OVERFLOW-4]].
: [[#scroll-navigation|Scroll navigation controls]]
::
This section defines stylable scroll navigation controls
with defined user interactions and accessible labels,
and pseudo-classes which automatically construct them.
: [[#fragmentation|Redirection of Overflow]]
::
This section defines a highly experimental, exploratory new model
for handling overflow by redirecting it into newly-generated [=fragmentation containers=].
Note: At the time of writing, [[CSS-OVERFLOW-4]] is not completely finalized yet.
To avoid accidental divergences and maintenance overhead,
This specification is written as a delta specification over css-overflow Level 4.
Once the level 4 specification is final,
its content will be integrated into this specification,
which will then replace it.
Until then, this specification only contains additions and extensions to level 4.
<h2 id="scroll-navigation">
Scroll navigation controls</h2>
<h3 id="scroll-markers">
Scroll markers</h3>
A <dfn>scroll marker</dfn> is any element or pseudo-element with a [=scroll target=].
An element or pseudo-element's <dfn>scroll target</dfn> is the {{Element}} indicated by the [=scroll marker=].
Which elements are [=scroll markers=], and what their [=scroll targets=] are, is host-language defined.
The [[html#the-a-element|HTML <a> element]] and [[svg2#Links|SVG <a> element]]
are [=scroll markers=],
whose [=scroll target=] is the <l spec=html>[=indicated part=]</l>.
While these navigational links can be created today,
there is little feedback to the user regarding the current content being viewed, and
the interaction model does not match the expectations of many modern accessible UI components.
This specification adds a mechanism for creating groups of [=scroll marker=]s,
and for automatically creating '::scroll-marker'' pseudo-elements.
Within each group, the active marker reflects the current scroll position,
and can be styled to give the user an indication of which section they are in.
Use cases include a table of contents with links to relevant contents,
markers for scrolling carousel pages,
and scrollable tab panels.
Issue: Add images representing these examples.
<h4 id="scroll-marker-grouping">
Scroll marker grouping</h4>
An element with a <a href="https://open-ui.org/components/focusgroup.explainer/">focusgroup</a> attribute defines a <dfn>scroll marker group container</dfn>
having a <dfn>scroll marker group</dfn> containing all of the [=scroll marker=] elements for which this is the nearest ancestor [=scroll marker group container=].
Issue: The grouping of markers for scroll progress tracking should be separated from opting into focusgroup focus behavior.
A ''::scroll-marker-group'' pseudo-element is the [=scroll marker group container=] for its contained ''::scroll-marker'' pseudo-elements, which form a [=scroll marker group=] together.
<h4 id="scroll-marker-group">
The 'scroll-marker-group' property</h4>
<pre class=propdef>
Name: scroll-marker-group
Value: none | before | after
Initial: none
Applies to: [=scroll containers=]
Inherited: no
Computed value: specified value
Animation Type: discrete
Canonical Order: per grammar
</pre>
The 'scroll-marker-group' property specifies whether the [=scroll container=] should have a '::scroll-marker-group' pseudo-element created,
and its position relative to the scroll container.
<dl dfn-type=value dfn-for=scroll-marker-group>
<dt><dfn>none</dfn>
<dd>
The [=scroll container=] does not create a '::scroll-marker-group' pseudo-element.
<dt><dfn>before</dfn>
<dd>
The [=scroll container=] generates a ''::scroll-marker-group'' pseudo-element
whose box is an immediate preceding sibling to its [=originating element=].
<dt><dfn>after</dfn>
<dd>
The [=scroll container=] generates a ''::scroll-marker-group'' pseudo-element
whose box is an immediate following sibling to its [=originating element=].
</dl>
<h4 id="scroll-marker-group-pseudo">The ''::scroll-marker-group'' pseudo-element</h4>
The <dfn selector>::scroll-marker-group</dfn> [=fully stylable pseudo-element=]
is generated by a [=scroll container=] element
having a computed 'scroll-marker-group' property that is not 'none',
adjacent to the [=scroll container=].
The following additions are recommended for the default UA stylesheet
to ensure that the generation of scroll marker pseudo-elements does not invalidate the layout of the site:
<pre class="lang-css">
/* The generation of ::scroll-marker pseudo-elements shouldn't
* invalidate layout outside of this pseudo-element. */
::scroll-marker-group { contain: size !important; }
</pre>
The 'scroll-marker-group' implicitly behaves as a single focusable component,
establishing a <a href="https://open-ui.org/components/focusgroup.explainer/">focusgroup</a>.
<h4 id="scroll-marker-pseudo">The ''::scroll-marker'' pseudo-element</h4>
Similar to ''::before'' and ''::after'', all elements can have a ''::scroll-marker'' pseudo-element,
which is collected into the ''::scroll-marker-group'' of the nearest [=scroll container=] ancestor,
and scrolls to the element when activated.
When the computed 'content' value of a <dfn selector>::scroll-marker</dfn> pseudo-element is not 'none'
and its nearest ancestor [=scroll container=] [=scroll container=] has a computed 'scroll-marker-group' property that is not 'none',
the pseudo-element generates a box attached as a child of the ''::scroll-marker-group'' pseudo-element's generated box
on its nearest ancestor [=scroll container=].
These boxes are added in the [=tree order=] of their <a>originating element</a>.
These pseudo-elements have an indicated [=scroll target=] of their <a>originating element</a>.
They behave as an element with a <a href="https://html.spec.whatwg.org/multipage/interaction.html#attr-tabindex"><code>tabindex</code></a> of '-1',
making them focusable within their '::scroll-marker-group' either by arrow key navigation within the group,
or via the tab key when currently active or when no other ''::scroll-marker'' is active and this is the first marker in the group,
ensuring the group has a <a href="https://open-ui.org/components/focusgroup.explainer/#guaranteed-tab-stop">guaranteed tab stop</a>.
<h4 id="active-scroll-marker">
Selecting The Active Scroll Marker: the '':target-current'' pseudo-class</h4>
Exactly one [=scroll marker=] within each [=scroll marker group=] is determined to be active at a time.
Such "active" [=scroll markers=] match the <dfn selector>:target-current</dfn> pseudo-class.
<div class='example'>
The following snippet shows how the link to the currently scrolled section can be highlighted:
<pre highlight=css>
a:target-current {
font-weight: bold;
}
</pre>
</div>
A scrolling operation might animate towards a particular position
(e.g. scrollbar arrow clicks, arrow key presses, "behavior: smooth" programmatic scrolls)
or might directly track a user’s input
(e.g. touch scrolling, scrollbar dragging).
In either case, the user agent chooses an 'eventual scroll position' to which the scroller
will reach. This ensures that the relevant marker is activated immediately.
This 'eventual scroll position' is used to determine the active marker within each [=scroll marker group=].
Since markers themselves may represent just the start of the content (e.g. headers), we consider the active marker to be the first one which we are at or beyond the scroll position of.
<div algorithm="scroll tracking">
Whenever a [=scroll container=] is scrolled, or layout changes the scroll position, the user agent must run these steps to determine the active marker:
1. Let <var>position</var> be the 'eventual scroll position' for the scrolling operation.
1. For each focusgroup <var>group</var> containing one or more [=scroll marker control=] elements whose [=scroll target=] is a descendant of [=scroll container=]:
1. Let <var>markers</var> be all of the [=scroll marker control=] elements which are a part of the [=scroll marker group=] for the [=scroll container=].
1. Let <var>targets</var> be the [=scroll target=]s of |markers|, associated with the item of |markers| they came from, and sorted in [=tree order=].
1. For each |target| in |targets|, <a>determine the scroll-into-view position</a> of |target|, storing this as the associated |target position| of |target|.
1. Let |selected target| be the largest-indexed item of |targets|
whose associated |target position| is equal to or before |position| in both the block and inline axises in the current writing mode direction of the [=scroll container=].
Issue: When the next marker is closer to being aligned than the previous we should use the next marker, in a manner similar to mandatory snap point selection.
1. : If there is no such item,
::
Set the active state of all |markers| in the |group| to false and return.
1. Let |selected marker| be the marker associated with |selected target|.
If multiple items of |markers| are associated with |selected target|,
set |selected marker| to be the marker that is earliest in tree order.
1. Set the active state of |selected marker| to true.
1. : If the active element was the <a href="https://open-ui.org/components/focusgroup.explainer/#last-focused-memory">last-focused element</a> of the |group|,
::
Focus |selected marker|
1. Set the <a href="https://open-ui.org/components/focusgroup.explainer/#last-focused-memory">last-focused element</a> of the |group| to |selected marker|.
1. Set the active state of all other |markers| in the |group| to false.
</div>
<h4 id="scroll-marker-activation">Activation behavior</h4>
<div algorithm="scrollTargetElement activation">
When a [=scroll marker=] with a non-null [=scroll target=] is activated by explicit invocation or arrow key focus:
1. Let <var>element</var> be the [=scroll target=] of the control.
1. Let <var>block</var> be "<code>start</code>".
1. Let <var>inline</var> be "<code>start</code>".
1. <a lt='scroll a target into view'>Scroll the element into view</a> with <var>behavior</var>, <var>block</var>, and <var>inline</var>.
1. : If the activation was triggered by invocation
::
1. <a spec=html>Follow the hyperlink</a> updating the URL, however retain focus on the marker element.
Note: If the user tabs away the focus behavior will ensure they tab into the relevant content.
</div>
<h4 id="scroll-target-focus">Focus behavior</h4>
When a [=scroll marker control=] is activated,
the next tabindex-ordered focus navigation will focus the [=scroll target=] if it is focusable,
otherwise, it will find the next focusable element from this <var>element</var> as though it were focused.
<h3 id="scroll-buttons">
Scroll Buttons</h3>
The <dfn>::scroll-button( <<scroll-button-direction>> )</dfn> pseudo-elements
are generated on [=scroll containers=]
when their computed 'content' value is not ''content/none''.
They generate boxes as if they were immediate children
of their [=originating element=],
with content as specified by 'content'.
They exist after their [=originating element's=] ''::scroll-marker-group'' pseudo-element
(when used with ''scroll-marker-group: before'')
and before its ''::before'' pseudo-element,
if those exist.
Four distinct ''::scroll-button()'' pseudo-elements can exist on a [=scroll container=],
each associated with a [=logical=] direction,
based on their [=originating element's=] [=writing mode=]:
in order, [=block-start=], [=inline-start=], [=block-end=], and [=inline-end=].
The ''::scroll-button()'' pseudo-elements are both focusable and activatable by default,
with their activation behavior being to scroll their [=originating element=]
by one "page" in their associated direction,
similar to pressing PgUp/PgDn keys.
<span class=note>(Usually, this will be about 85% of the [=scrollport=] size.)</span>
Like those keys, this scroll has an [=intended direction and end position=].
Issue: [[CSSOM-VIEW-1]] will grow a "scroll by pages" algorithm,
at which point this can reference that.
See <a href="https://github.com/w3c/csswg-drafts/issues/10914#issuecomment-2380139977">
the resolution on issue #10914</a>.
Issue: Is this the best order for the buttons?
This does match the usual ordering of logical directions in CSS,
but similar buttons created manually
are usually either ordered with verticals and horizontals each grouped together
(aka block-start/block-end then inline-start/inline-end)
or the verticals "around" the horizontals
(aka block-start/inline-start/inline-end/block-end).
The four ''::scroll-button()'' pseudo-elements are individually selected
by the selector's argument.
The possible <dfn><<scroll-button-direction>></dfn> values are:
<dl dfn-type=value dfn-for="<scroll-button-direction>, ::scroll-button()">
: <dfn>up</dfn>
: <dfn>down</dfn>
: <dfn>left</dfn>
: <dfn>right</dfn>
:: Selects the ''::scroll-button()' corresponding to the given physical direction.
: <dfn>block-start</dfn>
: <dfn>block-end</dfn>
: <dfn>inline-start</dfn>
: <dfn>inline-end</dfn>
:: Selects the indicated ''::scroll-button()'' pseudo-element.
: <dfn>prev</dfn>
::
Selects either the [=block-start=] or [=inline-start=] ''::scroll-button()'',
whichever's axis has more "scrollable pages" in the [=originating element=]:
the [=originating element's=] [=scrollable overflow height=]
divided by its [=scrollport=] height,
or the same but for widths.
If both dimensions are equally sized,
selects the [=block-start=] ''::scroll-button()''.
<p class=example>
For example, say the [=originating element=]
was 800px wide and 500px tall,
while its [=scrollable overflow area=]
was 1200px wide and 1000px tall.
The horizontal scrolling thus represents 1.5 "pages" (1200/800),
while the vertical scrolling represents 2 "pages" (1000/500),
so (assuming the element is in English)
the ''::scroll-button(prev)'' selector
would select the [=block-start=] button.
: <dfn>next</dfn>
:: Identical to ''::scroll-button()/prev'',
except it selects the [=block-end=] or [=inline-end=] ''::scroll-button()'' instead.
</dl>
Issue: Do we want to add some multi-button keywords
to make it easier to style several buttons the same way?
In particular, <css>all</css> is probably useful,
but maybe also <css>horizontal</css>/<css>vertical</css>/<css>block</css>/<css>inline</css>.
The ''::scroll-button()''s are [=fully stylable pseudo-elements=]: there is no restriction on what properties apply to them.
Issue: The UA stylesheet needs to specify that ''::scroll-button()''s
are styled identically to the <{button}> element.
<h3 id=focus-order>
Focus Navigation Order</h3>
The above features generate several focusable pseudo-elements.
While these pseudo-elements have a defined position in the element tree,
this isn't an optimal position for focus navigation
(aka "tab order")
for these controls.
Instead,
focus navigation between a [=scroll container=]
and the various pseudo-elements defined in this section
goes in the following order:
1. The ''::scroll-marker-group'' pseudo-elements of the [=scroll container=],
if it is set to ''scroll-marker-group: before''.
Note: The individual ''::scroll-marker'' pseudo-elements
generated by the [=scroll containers=] descendants
are reparented underneath this ''::scroll-marker-group'',
and navigated together as a "focus group".
2. The ''::scroll-button()'' pseudo-elements,
in the order they're defined as existing in.
3. The [=scroll container=] itself,
and its contents,
in the normal focus order they would be in.
4. The ''::scroll-marker-group'' pseudo-elements of the [=scroll container=],
if it is set to ''scroll-marker-group: after''.
<h2 id="fragmentation" class=no-num>
Appendix A: Redirection of Overflow</h2>
<!-- Abstract: This module contains the features of CSS relating to new mechanisms of overflow handling in visual media (e.g., screen or paper). In interactive media, it describes features that allow the overflow from a fixed size container to be handled by pagination (displaying one page at a time). It also describes features, applying to all visual media, that allow the contents of an element to be spread across multiple fragments, allowing the contents to flow across multiple regions or to have different styles for different fragments. -->
ISSUE: This section is highly experimental.
It documents current attempts
at extending the capabilities of the 'continue' property
to solve additional use cases.
However, it does not currently have consensus.
It is presented here to encourage discussion,
but non-experimental implementation is not recommended.
<p>
In CSS Level 1 [[CSS1]], placing more content than would fit
inside an element with a specified size
was generally an authoring error.
Doing so caused the content to extend
outside the bounds of the element,
which would likely cause
that content to overlap with other elements.
</p>
<p>
CSS Level 2 [[CSS2]] introduced the 'overflow' property,
which allows authors to have overflow be handled by scrolling,
which means it is no longer an authoring error.
It also allows authors to specify
that overflow is handled by clipping,
which makes sense when the author's intent
is that the content not be shown.
This was further refined in the CSS Overflow Module Level 3 [[CSS-OVERFLOW-3]].
</p>
<p>
However, scrolling is not the only way
to present large amounts of content,
and may even not be the optimal way.
After all, the codex replaced the scroll
as the common format for large written works
because of its advantages.
</p>
<p>
This specification introduces
a mechanism for Web pages to specify
that an element of a page should handle overflow
through pagination rather than through scrolling.
</p>
<p>
This specification also extends the concept of overflow
in another direction.
Instead of requiring that authors specify a single area
into which the content of an element must flow,
this specification allows authors to specify multiple fragments,
each with their own dimensions and styles,
so that the content of the element can flow from one to the next,
using as many as needed to place the content without overflowing.
</p>
<p>
In both of these cases, implementations must
break the content in the block-progression dimension.
Implementations must do this is described
in the CSS Fragmentation Module [[!CSS-BREAK-3]].
</p>
<h3 id="channelling-overflow">
Channeling Overflow: the 'continue' property</h3>
The 'continue' property gives authors the ability
to request that content that does not fit inside an element
be fragmented (in the sense of [[!CSS-BREAK-3]]),
and provides alternatives
for where the remaining content should continue.
Notably, this property explains traditional pagination,
and extends it further.
<pre class="propdef partial">
Name: continue
New Values: overflow | paginate | fragments
Initial: auto
Applies to: block containers [[!CSS2]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
Inherited: no
Percentages: N/A
Computed value: see below
Animation type: discrete
</pre>
Issue: The naming of this property and its values is preliminary.
This was initially proposed as
"fragmentation: auto | none | break | clone | page"
in <a href="https://lists.w3.org/Archives/Public/www-style/2015Jan/0357.html">https://lists.w3.org/Archives/Public/www-style/2015Jan/0357.html</a>,
and there is not yet wide agreement as to which naming is better.
Issue: This property is meant to generalize and replace 'region-fragment'.
Once it is sufficiently stable in this specification,
'region-fragment' should be removed from the regions specification in favor of this.
Note: ''continue: fragments'' replaces "overflow:fragments"
from earlier versions of this specification,
while ''continue: paginate'' replaces "overflow: paged-x | paged-y | paged-x-controls | paged-y-controls"
<dl dfn-for="continue" dfn-type="value">
<dt>auto
<dd>''continue/auto'' may only occur as a computed value
if the element is a <a spec="css-regions">CSS Region</a>
other than the last one in a <a spec="css-regions">region chain</a>.
Content that doesn't fit is pushed to the next region of the chain.
In all other cases, ''continue/auto'' computes to one of the other values.
Issue: this is different from the definition in [[css-overflow-4#continue]],
where the specified value is the computed value.
Which is model better?
<dt><dfn>overflow</dfn>
<dd>Content that doesn't fit overflows, according to the 'overflow' property
<dt><dfn>paginate</dfn>
<dd>Content that doesn't fit paginates.
This creates a paginated view inside the element
similar to the way that 'overflow: scroll' creates a scrollable view.
See <a href="#paginated-overflow">paginated overflow</a>
Note: Print is effectively "continue: paginate" on the root.
<dt><dfn>fragments</dfn>
<dd>content that doesn't fit causes the element to copy itself and continue laying out.
See <a href="#fragment-overflow">fragment overflow</a>.
</dl>
The computed value of the 'continue' for a given element or pseudo element is determined as follow:
<ol>
<li>On elements or pseudo elements with <a>layout containment</a> (see [[!CSS-CONTAIN-1]]),
if the specified value is ''continue/auto'' or ''continue/fragments''
then the computed value is ''continue/overflow''.
<li>Otherwise, if the specified value is ''continue/auto''
<ol>
<li>On a <a spec="css-regions">CSS Region</a> other than the last one in a <a spec="css-regions">region chain</a>,
the computed value is ''continue/auto''
<li>On a page
the computed value is ''continue/paginate''
<li>On a <a>fragment box</a>
the computed value is ''continue/fragments''
<li>Otherwise, the computed value is ''continue/overflow''
</ol>
<li>Otherwise, if the specified value is ''continue/fragments''
<ol>
<li>On a page
the computed value is ''continue/paginate''
<li>Otherwise, the computed value is the specified value
</ol>
<li>In all other cases, the computed value is the specified value
</ol>
Issue: If we introduce a pseudo element that can select columns in a multicol,
we would need to specify that auto computes to auto on it,
or introduce a new value and have auto compute to that
(but what would that value compute to on things that aren't columns?).
Note: For background discussions leading to this property, see these threads:
<a href="http://lists.w3.org/Archives/Public/www-style/2012May/1197.html">discussion of overflow, overflow-x, overflow-y and overflow-style</a> and
<a href="https://lists.w3.org/Archives/Public/www-style/2015Jan/0357.html">proposal for a fragmentation property</a>
<h3 id="paginated-overflow">
Paginated overflow</h3>
This section introduces and defines the meaning of the ''continue/paginate'' value of the 'continue' property.
Issue: Write this section
Issue: Pages should be possible to style with @page rules. How does that work for nested pages?
<div class="issue">
Should traditional pagination (e.g. when printing)
be expressed through some magic in the computed value of ''continue/auto'',
or by inserting this in the UA stylesheet:
<pre><code class="lang-css">
@media (overflow-block: paged), (overflow-block: optional-paged) {
:root {
continue: paginate;
}
}
</code></pre>
</div>
Issue: Traditional pagination (e.g. when printing) assumes that
:root is contained in the page box,
rather than having the page box be a pseudo element child of :root.
Can we work around that using something similar to fragment boxes?
Or maybe by having a fragment box (reproducing :root) inside a page box inside :root?
Issue: How does the page box model work when it is a child of a regular css box?
Issue: The initial proposal in [[CSS3GCPM]] and implementation from Opera
used 4 values instead of ''continue/paginate'':
"paged-x | paged-y | paged-x-controls | paged-y-controls".
Should this property also include these values,
or are they better handled as separate properties?
(e.g.: "pagination-layout: auto | horizontal | vertical", "pagination-controls: auto | none")
Issue: Ability to display N pages at once
rather than just one page at once?
Could this be a value of "pagination-layout", such as:
"pagination-layout: horizontal 2;"
Issue: Brad Kemper has proposed a model for combining pagination and
fragment overflow, which also deals with displaying multiple pages.
<a href="http://www.w3.org/mid/FF1704C5-D5C1-4D6F-A99D-0DD094036685@gmail.com">http://www.w3.org/mid/FF1704C5-D5C1-4D6F-A99D-0DD094036685@gmail.com</a>
<p class="issue">
The current implementation of paginated overflow uses
the overflow/overflow-x/overflow-y properties
rather than the overflow-style property as proposed
in the [[CSS3GCPM]] draft
(which also matches the [[CSS3-MARQUEE]] proposal).
or the 'continue' property as described here.
</p>
<h3 id="fragment-overflow">
Fragmented Overflow</h2>
This section introduces and defines the meaning of
the ''continue/fragments'' value of the 'continue' property.
<p>
When the computed value of 'continue' for an element is ''continue/fragments'',
and implementations would otherwise have created a box for the element,
then implementations must create a sequence of <dfn>fragment box</dfn>es
for that element.
(It is possible for an element with ''continue: fragments''
to generate only one <a>fragment box</a>.
However, if an element's computed 'continue' is not ''continue/fragments'',
then its box is not a <a>fragment box</a>.)
Every <a>fragment box</a> is a fragmentation container,
and any overflow
that would cause that fragmentation container to fragment
causes another <a>fragment box</a> created as a next sibling
of the previous one.
<span class="issue">Or is it as though it's a next sibling of
the element? Need to figure out exactly how this interacts with
other box-level fixup.</span>
Additionally, if the <a>fragment box</a> is also
a multi-column box (as defined in [[!css-multicol-1]]
<span class="issue">though it defines <i>multi-column container</i></span>)
any content that would lead to the creation of <a>overflow columns</a> [[!css-multicol-1]]
instead is flown into an additional fragment box.
However, fragment boxes may themselves be broken
(due to fragmentation in a fragmentation context outside of them,
such as pages, columns, or other fragment boxes);
such breaking leads to fragments of the same fragment box
rather than multiple fragment boxes.
(This matters because fragment boxes may be styled by their index;
such breaking leads to multiple fragments of a fragment box
with a single index.
This design choice is so that
breaking a fragment box across pages does not break
the association of indices to particular pieces of content.)
<span class="issue">Should a forced break that breaks to
an outer fragmentation context cause a new fragment of a single
fragment box or a new fragment box?</span>
<span class="issue">Should we find a term other than
<a>fragment box</a> here to make this a little less confusing?</span>
</p>
<p class="issue">
What if we want to be able to style the pieces of an element
split within another type of fragmentation context?
These rules prevent ever using ''::nth-fragment()'' for that,
despite that the name seems the most logical name for such a feature.
</p>
<div class="example">
<table class="source-demo-pair"><tr><td>
<pre><code highlight="html"><!DOCTYPE HTML>
<title>Breaking content into
equal-sized cards</title>
<style>
.in-cards {
continue: fragments;
width: 13em;
height: 8em;
padding: 4px;
border: medium solid blue;
margin: 6px;
font: medium/1.3 Times New
Roman, Times, serif;
}
</style>
<div class="in-cards">
In this example, the text in the div
is broken into a series of cards.
These cards all have the same style.
The presence of enough content to
overflow one of the cards causes
another one to be created. The second
card is created just like it's the
next sibling of the first.
</div></code></pre></td><td>
<div class="in-cards-demo">In this example, the text in the<br>div is broken into a series of<br>cards. These cards all have the<br>same style. The presence of<br>enough content to overflow<br>one of the cards causes another</div>
<div class="in-cards-demo">one to be created. The second<br>card is created just like it's the<br>next sibling of the first.</div>
</td></tr></table>
</div>
<div class="example">
Authors may wish to style the opening lines of an element
with different styles
by putting those opening lines in a separate fragment.
However, since it may be difficult to predict the exact height
occupied by those lines
in order to restrict the first fragment to that height,
it is more convenient to use the 'max-lines' property,
which forces a fragment to break
after a specified number of lines.
This forces a break after the given number of lines
contained within the element or its descendants,
as long as those lines are in the same block formatting context.
<table class="source-demo-pair"><tr><td>
<pre><code highlight="html"><!DOCTYPE HTML>
<style>
.article {
continue: fragments;
}
.article::first-letter {
font-size: 2em;
line-height: 0.9;
}
.article::nth-fragment(1) {
font-size: 1.5em;
max-lines: 3;
}
.article::nth-fragment(2) {
column-count: 2;
}
</style>
<div class="article">
<i>...</i>
</div></code></pre></td><td>
<div class="article-max-lines-demo one">The max-lines property allows<br>authors to use a larger font for the first<br>few lines of an article. Without the</div>
<div class="article-max-lines-demo two">max-lines property, authors<br>might have to use the<br>'height' property instead, but<br>that would leave a slight gap<br>if the author miscalculated<br>how much height a given<br>number of lines would<br>occupy (which might be</div>
<div class="article-max-lines-demo three">particularly hard if the author<br>didn't know what text would<br>be filling the space, exactly<br>what font would be used, or<br>exactly which platform's font<br>rendering would be used to<br>display the font).</div>
</td></tr></table>
</div>
<p class="issue">
We should specify that ''continue: fragments'' does not apply
to at least some table parts,
and perhaps other elements as well.
We need to determine exactly which ones.
</p>
<p class="issue">
This specification needs to say which type of
fragmentation context is created
so that it's clear which values of the 'break-*' properties
cause breaks within this context.
We probably want ''break-*: region'' to apply.
</p>
<p class="issue">
This specification needs a processing model
that will apply in cases where the layout containing the
fragments has characteristics that use the intrinsic size of the fragments
to change the amount of space available for them,
such as [[CSS3-GRID-LAYOUT]].
There has already been some work on such a processing model
in [[CSS-REGIONS-1]],
and the work done on a model there,
and the editors of that specification,
should inform what happens in this specification.
</p>
<h3 id="fragment-styling">Fragment styling</h3>
<h4 id="fragment-pseudo-element">The ''::nth-fragment()'' pseudo-element</h4>
<p>
The <dfn selector>::nth-fragment()</dfn> pseudo-element
is a pseudo-element
that describes some of the <a>fragment box</a>es generated by an element.
The argument to the pseudo-element takes the same syntax
as the argument to the :nth-child() pseudo-class
defined in [[!SELECT]], and has the same meaning
except that the number is relative to
<a>fragment box</a>es generated by the element
instead of siblings of the element.
</p>
<p class="note">
Selectors that allow addressing fragments
by counting from the end rather than the start
are intentionally not provided.
Such selectors would interfere with determining
the number of fragments.
</p>
<p class="issue">
Depending on future discussions,
this ''::nth-fragment(<var>an+b</var>)'' syntax
may be replaced with
the new ''::fragment:nth(<var>an+b</var>)'' syntax.
</p>
<h4 id="style-of-fragments">Styling of fragments</h4>
<p class="issue">
Should this apply to continue:fragments only,
or also to continue:paginate?
(If it applies,
then stricter property restrictions would be needed
for continue:paginate.)
</p>
<p>
In the absence of rules with ''::nth-fragment()'' pseudo-elements,
the computed style for each <a>fragment box</a>
is the computed style for the element
for which the <a>fragment box</a> was created.
However, the style for a <a>fragment box</a> is also influenced
by rules whose selector's <a>subject</a> [[!SELECT]]
has an ''::nth-fragment()'' pseudo-element,
if the 1-based number of the <a>fragment box</a> matches
that ''::nth-fragment()'' pseudo-element
and the selector (excluding the ''::nth-fragment()'' pseudo-element)
matches the element generating the fragments.
</p>
<p>
When determining the style of the <a>fragment box</a>,
these rules that match the fragment pseudo-element
cascade together with the rules that match the element,
with the fragment pseudo-element adding the specificity
of a pseudo-class to the specificity calculation.
<span class="issue">Does this need to be specified in
the cascading module as well?</span>
</p>
<div class="example">
<table class="source-demo-pair"><tr><td>
<pre><code highlight="html"><!DOCTYPE HTML>
<style>
.bouncy-columns {
continue: fragments;
width: 6em;
height: 10em;
float: left;
margin: 1em;
font: medium/1.25 Times New
Roman, Times, serif;
}
.bouncy-columns::nth-fragment(1) {
background: aqua; color: black;
transform: rotate(-3deg);
}
.bouncy-columns::nth-fragment(2) {
background: yellow; color: black;
transform: rotate(3deg);
}
</style>
<div class="bouncy-columns">
<i>...</i>
</div></code></pre></td><td>
<div class="bouncy-columns-demo one">In this<br>example, the<br>text in the div<br>is broken into<br>a series of<br>columns. The<br>author<br>probably</div>
<div class="bouncy-columns-demo two">intended the<br>text to fill two<br>columns. But<br>if it happens to<br>fill three<br>columns, the<br>third column is<br>still created. It</div>
<div class="bouncy-columns-demo">just doesn't<br>have any<br>fragment-specific<br>styling because<br>the author<br>didn't give it<br>any.</div>
</td></tr></table>
</div>
<p>
Styling an ''::nth-fragment()'' pseudo-element with the 'continue'
property does take effect;
if a <a>fragment box</a> has a
computed value of 'continue' other than ''fragments''
then that fragment box is the last fragment.
However, overriding 'continue' on the first fragment
does not cause the <a>fragment box</a> not to exist;
whether there are fragment boxes at all is determined by
the computed value of overflow for the element.
</p>
<p>
Styling an ''::nth-fragment()'' pseudo-element with the 'content'
property has no effect;
the computed value of 'content' for the fragment box
remains the same as the computed value of content for the element.
</p>
<p>
Specifying ''display: none'' for a <a>fragment box</a> causes
the fragment box with that index not to be generated.
However, in terms of the indices
used for matching ''::nth-fragment()'' pseudo-elements
of later fragment boxes,
it still counts as though it was generated.
However, since it is not generated, it does not contain any content.
</p>
<p>
Specifying other values of 'display', 'position',
or 'float' is permitted, but is not allowed to change
the <a>inner display type</a>.
(Since 'continue' only
applies to block containers, flex containers, and grid containers).
<span class="issue">Need to specify exactly how this works</span>
</p>
<p>
To match the model for other pseudo-elements
where the pseudo-elements live inside their corresponding element,
declarations in ''::nth-fragment()'' pseudo-elements override
declarations in rules without the pseudo-element.
The relative priority within such declarations is determined
by normal cascading order (see [[!CSS2]]).
</p>
<p>
Styles specified on ''::nth-fragment()'' pseudo-elements
do affect inheritance to content within the <a>fragment box</a>.
In other words, the content within the <a>fragment box</a> must
inherit from the fragment box's style (i.e., the pseudo-element style)
rather than directly from the element.
This means that elements split between fragment boxes may
have different styles for different parts of the element.
</p>