-
Notifications
You must be signed in to change notification settings - Fork 708
/
Copy pathOverview.bs
1728 lines (1457 loc) · 67.4 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 3
Status: ED
Work Status: Revising
ED: https://drafts.csswg.org/css-overflow-3/
Shortname: css-overflow
Group: csswg
Level: 3
TR: https://www.w3.org/TR/css-overflow-3/
Previous version: https://www.w3.org/TR/2020/WD-css-overflow-3-20200603/
Previous version: https://www.w3.org/TR/2018/WD-css-overflow-3-20180731/
Previous version: https://www.w3.org/TR/2016/WD-css-overflow-3-20160531/
Previous version: https://www.w3.org/TR/2013/WD-css-overflow-3-20130418/
Editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/, w3cid 15393
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Florian Rivoal, On behalf of Bloomberg, http://florian.rivoal.net/, w3cid 43241
Abstract: This module contains the features of CSS relating to scrollable overflow handling in visual media.
This level is focused on completing a precise specification for the existing overflow features,
including the 'overflow' property and its longhands;
'line-clamp', its longhands, and its legacy pre-standard syntax;
and the 'text-overflow' property.
A few additional features introduced in support of [[CSS-CONTAIN-1]] and [[CSS-CONTAIN-2]] are also defined:
''overflow: clip'' and the 'overflow-clip-margin' property.
Ignored Vars: B, P
Ignored Terms: padding edge, viewport, line box, flex order, -webkit-box-orient
At Risk: the 'max-lines' property
Status Text:
The description of 'overflow' and its longhands is considered
significantly more complete and correct than previous working drafts
or than [[CSS2]],
but a few questions and issues remain open.
''overflow: clip' and 'overflow-clip-margin' are rather new,
and lack implementation experience.
'text-overflow' is stable,
is unchanged form its earlier definition in [[CSS-UI-3]].
While not yet fully validated by implementation experience,
the design of 'line-clamp' and its longhands is considered roughly complete.
Earlier versions of this specification included experimental new ideas
for handling overflow by creating new boxes using fragmentation.
These ideas are not abandoned; they are merely <a href="https://drafts.csswg.org/css-overflow-4/">deferred until Level 4</a>.
Work will resume on fragmented overflow once this level stabilizes completed.
</pre>
<!-- FIXME: Regressions from bikeshed conversion: -->
<!-- - Value lines in propdef tables no longer link to #values. -->
<!-- - no longer says "Test suite: none yet" -->
<!-- - Abstract has the most introductory sentence last -->
<!-- FIXME: other bikeshed issues -->
<pre class="link-defaults">
spec:css-transforms-2; type:property; text:transform-style
type: dfn; spec:css-multicol-1; text:overflow column
type: dfn; spec:css-transforms-2; text:3d rendering context
spec:css-writing-modes-4; type:dfn; text:end
spec:css-pseudo-4; type:selector; text:::first-line
spec:css-pseudo-4; type:selector; text:::first-letter
spec:css-display-3; type:property; text:display
spec:css-sizing-3; type:property; text:max-height
spec:css-box-4; type:dfn; text:content area
spec:mediaqueries-3; type:value; text:print
</pre>
<!-- FIXME: the break-* link doesn't actually work! -->
<pre class="anchors">
url: https://www.w3.org/TR/2008/CR-css3-marquee-20081205/#the-overflow-style; type: property; text: overflow-style;
url: https://www.w3.org/TR/CSS22/visuren.html#positioned-element; type: dfn; spec:css2; text:positioned;
url: https://www.w3.org/TR/CSS22/visudet.html#strut-element; type: dfn; spec:css2; text:strut;
</pre>
<style>
.awesome-table td { padding: 5px; }
.awesome-table {
color: #000;
background: #fff;
margin: auto;
}
</style>
<h2 id="intro">
Introduction</h2>
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.
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 specification introduces the long-standing de-facto 'overflow-x' and 'overflow-y' properties,
adds a ''overflow/clip'' value,
and defines overflow handling more fully.
[Something something 'max-lines'.]
Note: This specification also reproduces the definition of the 'text-overflow' property
previously defined in [[CSS-UI-3]],
with no addition or modification,
in order to present 'text-overflow' and 'block-ellipsis' together.
<h3 id="values">
Value Definitions</h3>
This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
Value types not defined in this specification are defined in CSS Values & Units [[!CSS-VALUES-3]].
Combination with other CSS modules may expand the definitions of these value types.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the <a>CSS-wide keywords</a> as their property value.
For readability they have not been repeated explicitly.
<!--
████████ ██ ██ ████████ ████████ ██████
██ ██ ██ ██ ██ ██ ██ ██
██ ████ ██ ██ ██ ██
██ ██ ████████ ██████ ██████
██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██
██ ██ ██ ████████ ██████
-->
<h2 id="overflow-concepts">
Types of Overflow</h2>
CSS uses the term <dfn export>overflow</dfn> to describe
the contents of a box
that extend outside one of that box's edges
(i.e., its <i>content edge</i>, <i>padding edge</i>,
<i>border edge</i>, or <i>margin edge</i>).
The term might be interpreted as
elements or features that cause this overflow,
the non-rectangular region occupied by these features,
or, more commonly, as
the minimal rectangle that bounds that region.
A box's overflow is computed based on the layout and styling of the box itself
and of all descendants whose <a>containing block chain</a>
includes the box.
In most cases, <a>overflow</a>
can be computed for any box
from the bounds and properties of that box itself,
plus the <a>overflow</a>
of each of its children.
However, this is not always the case; for example,
when ''transform-style: preserve-3d'' [[CSS3-TRANSFORMS]] is used on
some of the children, any of their descendants with
''transform-style: preserve-3d'' must also be examined.
There are two different types of overflow,
which are used for different purposes by the UA:
<ul>
<li><a>ink overflow</a>
<li><a>scrollable overflow</a>
</ul>
<h3 id="ink">Ink Overflow</h3>
The <dfn export>ink overflow</dfn> of a box
is the part of that box and its contents that
creates a visual effect outside of
the box's border box.
Ink overflow is the overflow of painting effects defined to not affect layout
or otherwise extend the <a>scrollable overflow area</a>,
such as <a href="https://www.w3.org/TR/css-backgrounds/#box-shadow">box shadows</a>,
<a href="">border images</a>,
<a href="https://www.w3.org/TR/css-text-decor-3/">text decoration</a>,
overhanging glyphs (with negative side bearings,
or with ascenders/descenders extending outside the em box),
<a href="https://www.w3.org/TR/css-ui-3/#outline-props">outlines</a>,
etc.
Since some effects in CSS (for example, the blurs in
'text-shadow' [[CSS-TEXT-3]] and 'box-shadow' [[CSS-BACKGROUNDS-3]],
which are theoretically infinite)
do not define what visual extent they cover, the extent
of the <a>ink overflow</a> is undefined.
The <dfn export lt="ink overflow region | ink overflow area">ink overflow area</dfn> is the non-rectangular area
occupied by the <a>ink overflow</a>
of a box and its contents,
and the <dfn export>ink overflow rectangle</dfn> is
the minimal rectangle whose axes are aligned to the box's axes
and that contains the <a>ink overflow area</a>.
Note that the <a>ink overflow rectangle</a> is a rectangle
in the box's coordinate system, but might be non-rectangular
in other coordinate systems due to transforms. [[CSS3-TRANSFORMS]]
<h3 id="scrollable">
Scrollable Overflow</h3>
The <dfn export>scrollable overflow</dfn> of a box is the
set of things extending outside of that box's padding edge
for which a scrolling mechanism needs to be provided.
The <dfn export lt="scrollable overflow region | scrollable overflow area">scrollable overflow area</dfn> is the non-rectangular region
occupied by the <a>scrollable overflow</a>, and the
<dfn export>scrollable overflow rectangle</dfn> is
the minimal rectangle whose axes are aligned to the box's axes
and that contains the <a>scrollable overflow area</a>.
The <a>scrollable overflow area</a> is the union of:
<ul>
<li>
The [=scroll container=]’s own [=padding box=].
<li>
All [=line boxes=] directly contained by the [=scroll container=].
<li>
The border boxes
of all boxes for which it is the containing block
and whose border boxes are positioned not wholly outside
its <a>block-start</a> or <a>inline-start</a> padding edges,
accounting for transforms by projecting each box onto
the plane of the element that establishes its <a>3D rendering context</a>.
[[!CSS3-TRANSFORMS]]
Issue: Is this description of handling transforms
sufficiently accurate?
Border boxes with zero area
do not affect the [=scrollable overflow area=].
<li>
The margin areas of <a>grid item</a> and <a>flex item</a> boxes
for which the box establishes a containing block.
The UA may <em>additionally</em> include
the margin areas of other boxes
for which the box establishes a containing block;
however,
the conditions under which such margin areas are included
is undefined in this level.
<span class="issue">This needs further testing and investigation;
is therefore deferred in this draft.</span>
<li>
The <a>scrollable overflow areas</a> of all of the above boxes
(including zero-area boxes
and accounting for transforms as described above),
provided they themselves have ''overflow: visible''
(i.e. do not themselves trap the overflow)
and that <a>scrollable overflow</a> is not already clipped
(e.g. by the 'clip' property or the 'contain' property).
Issue: should ''overflow: clip'' also clip the <a>scrollable overflow</a>
or should it remain a pure paint-time operation,
which would mean that scrollable overflow, while invisible, would still be scrollable.
Note: The 'mask-*' properties [[!CSS-MASKING-1]]
do not affect the <a>scrollable overflow area</a>.
Issue: Need to evaluate compat of honoring or ignoring 'clip' and 'clip-path'.
<li>
Additional padding added as necessary
to the end-side of the [=scrollable overflow rectangle=]
such that the box’s content can be scrolled to a position
that would satisfy the requirements of ''place-content: end'' alignment.
Note: This padding represents the box’s own padding
within the [=scrollable overflow rectangle=]
so that when its in-flow content is scrolled to the end,
there is padding between the end-edge of the content
and the border edge of the box.
It typically ends up being exactly the same size as the box's own padding,
except in a few cases, such as when an out-of-flow element,
or the visible overflow of a descendent,
is increasing the size of the [=scrollable overflow rectangle=].
Including this padding is optional for <a>block containers</a>
in the inline axis if 'align-content'
is ''justify-content/normal''.
Note: It's hoped that including including inline-axis padding here is Web-compatible
as that is a better behavior,
but investigation as to whether that can be done without breaking existing content has not yet concluded.
This clause is optional while under investigation.
Existing behavior in the inline axis is
<a href="https://www.software.hixie.ch/utilities/js/live-dom-viewer/?saved=8496">quirky</a>
<a href="https://www.software.hixie.ch/utilities/js/live-dom-viewer/?saved=8499">and</a>
<a href="https://www.software.hixie.ch/utilities/js/live-dom-viewer/?saved=9539">inconsistent</a>.
See <a href="https://github.com/w3c/csswg-drafts/issues/129">Issue 129</a>.
<!-- Make sure http://jsbin.com/qececaqilo/edit?html,css,output is clearly defined. -->
</ul>
<!--
Considerations:
* floats
* relpos
* margins that collapsed through to the edge of the scrollable area
* inline-side margins on both blocks and inlines
Notes:
The reasoning for only including border boxes is to minimize the cases
in which overflow scrolling is triggered; however once overflow is triggered,
excluding margins creates tight, uncomfortable edges
because the margin has been effectively truncated.
Testcase:
http://software.hixie.ch/utilities/js/live-dom-viewer/?%3C!DOCTYPE%20html%3E%0A%3Cstyle%3E%0A%20%20div%20%7B%20height%3A%2050px%3B%20overflow%3A%20scroll%3B%20border%3A%20solid%3B%20position%3A%20relative%3B%7D%0A%20%20p%20%7B%20margin%3A%2025px%3B%20background%3A%20red%3B%20%7D%0A%20%20q%20%7B%20position%3A%20absolute%3B%20background%3A%20blue%3B%20top%3A%2025px%3B%20margin%3A%2025px%3B%20%7D%0A%3C%2Fstyle%3E%0A%3Cdiv%3E%0A%20%20%3Cp%3ETest%20%3Cq%3ETest2%0A%3C%2Fdiv%3E
-->
Note: The <a>scrollable overflow rectangle</a> is always a rectangle
in the box's own coordinate system, but might be non-rectangular
in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].
This means scrollbars can sometimes appear when not actually necessary.
<!--
███████ ██ ██ ████████ ████████ ████████ ██ ███████ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██████ ████████ ██████ ██ ██ ██ ██ ██ ██ ███████ █████████
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██ ██
███████ ███ ████████ ██ ██ ██ ████████ ███████ ███ ███
-->
<h2 id="overflow-properties">
Scrolling and Clipping Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</h2>
These properties specify whether a box’s content
(including any <a>ink overflow</a>)
is clipped to its padding edge,
and if so,
whether it is a <dfn export>scroll container</dfn>
that allows the user to scroll clipped parts of its <a>scrollable overflow area</a>
into view.
The visual viewport of the <a>scroll container</a>
(through which the <a>scrollable overflow area</a> can be viewed)
coincides with its padding box,
and is called the <dfn export>scrollport</dfn>.
For convenience, a box’s <dfn export>nearest scrollport</dfn>
is the [=scrollport=] of its nearest [=scroll container=] ancestor.
<pre class=propdef>
Name: overflow-x, overflow-y
Value: visible | hidden | clip | scroll | auto
Initial: ''visible''
Applies to: block containers [[!CSS2]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
Inherited: no
Percentages: N/A
Computed value: as specified, except with ''visible''/''clip'' computing to ''overflow/auto''/''hidden'' (respectively) if one of 'overflow-x' or 'overflow-y' is neither ''visible'' nor ''clip''
Animation type: discrete
</pre>
The 'overflow-x' property specifies
the handling of <a>overflow</a> in the horizontal direction
(i.e., overflow from the left and right sides of the box),
and the 'overflow-y' property specifies the handling
of <a>overflow</a> in the vertical direction
(i.e., overflow from the top and bottom sides of the box).
<pre class="propdef shorthand">
Name: overflow
Value: [ visible | hidden | clip | scroll | auto ]{1,2}
Initial: visible
Applies to: block containers [[!CSS2]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
Inherited: no
Percentages: N/A
Computed value: see individual properties
Animation type: discrete
</pre>
The 'overflow' property is a shorthand property
that sets the specified values of 'overflow-x' and 'overflow-y' in that order.
If the second value is omitted, it is copied from the first.
Values have the following meanings:
<dl dfn-for="overflow, overflow-x, overflow-y" dfn-type="value">
<dt><dfn>visible</dfn>
<dd>
There is no special handling of overflow, that is,
the box’s content is rendered outside the box if positioned there.
The box is not a <a>scroll container</a>.
<dt><dfn>hidden</dfn>
<dd>
This value indicates that
the box’s content is clipped to its padding box
and that the UA must not provide any scrolling user interface
to view the content outside the clipping region,
nor allow scrolling by direct intervention of the user,
such as dragging on a touch screen
or using the scrolling wheel on a mouse.
However, the content must still be scrollable programmatically,
for example using the mechanisms defined in [[CSSOM-VIEW]],
and the box is therefore still a <a>scroll container</a>.
<dt><dfn>clip</dfn>
<dd>
This value indicates that
the box’s content is clipped to its [=overflow clip edge=]
and that no scrolling user interface should be provided by the UA
to view the content outside the clipping region.
In addition, unlike ''overflow: hidden''
which still allows programmatic scrolling,
''overflow: clip'' forbids scrolling entirely,
through any mechanism,
and therefore the box is not a <a>scroll container</a>.
Unlike ''hidden'', this value <strong>does not</strong> cause
the element to establish a new formatting context.
Note: Authors who also want the box to establish a formatting context
may use ''display: flow-root'' together with ''overflow: clip''.
<dt><dfn>scroll</dfn>
<dd>
This value indicates that
the content is clipped to the padding box,
but can be scrolled into view
(and therefore the box is a <a>scroll container</a>).
Furthermore, if the user agent uses a scrolling mechanism
that is visible on the screen (such as a scroll bar or a panner),
that mechanism should be displayed
whether or not any of its content is clipped.
This avoids any problem with scrollbars appearing
and disappearing in a dynamic environment.
When the target medium is ''@media/print'',
overflowing content may be printed;
it is not defined where it may be printed.
<dt><dfn>auto</dfn>
<dd>
Like ''overflow/scroll'' when the box has <a>scrollable overflow</a>;
like ''overflow/hidden'' otherwise.
Thus, if the user agent uses a scrolling mechanism
that is visible on the screen (such as a scroll bar or a panner),
that mechanism will only be displayed
if there is overflow.
</dl>
If the computed value of 'overflow' on a <a>block box</a>
is neither ''overflow/visible'' nor ''overflow/clip'' nor a combination thereof,
it [=establishes an independent formatting context=] for its contents.
If the computed value of the 'visibility' property is ''visibility/hidden''
(or ''visibility/collapse'' when it has the same effect as ''visibility/hidden''),
and 'overflow' is either ''overflow/scroll'' or ''overflow/auto'',
then:
* The user agent must not make any scrolling mechanism visible.
To the extent that the scrolling mechanism that would normally be visible
in the absence of ''visibility: hidden'' affects layout,
it continues to do so, but is not painted.
* As would be the case with ''overflow: hidden'',
scrolling directly triggered by user interactions is disabled,
but programmatic scrolling continues to take effect.
* The lack of interactive direct scrolling is enforced
even if the user interacts (e.g. with a mouse scrolling wheel)
with a descendent of the ''visibility: hidden'' [=scroll container=]
that is itself set to ''visibility: visible''.
Note: In the case where the [=scroll container=] (or one of its ancestors)
is the target of a graphical transform,
the UA might need to take this transform into account
when mapping user inputs to scrolling operations.
For instance, on a touch screen where the user scrolls
by directly dragging the content,
the transform would be expected to be taken into account
to match the direction of scrolling to the gesture.
On the other hand, other user inputs
(such as the Page Down key, or a 1D scroll wheel)
might be more naturally interpreted ignoring the transform.
Choosing the appropriate behavior for each scrolling mechanism
is the responsibility of the UA.
<h3 id="static-media">
Overflow in Print and Other Static Media</h3>
<p class="advisement">
Since scrolling is not possible in static media
(such as print)
authors should be careful to make content accessible in such media,
for example by using ''@media print, (update: none) { … }''
to adjust layout such that all relevant content
is simultaneously visible.
On <a>scroll containers</a> in non-interactive media
with an 'overflow' value of ''overflow/auto'' or ''overflow/scroll''
(but not ''overflow/hidden'')
UAs may display an indication of any scrollable overflow,
such as by displaying scrollbars
or an ellipsis.
Note: Not all [=paged media=] is non-interactive
for example, e-book readers page content,
but are interactive.
<h3 id="scrollbar-layout">
Scrollbars and Layout</h3>
When reserving space for a scrollar placed at the edge of an element's box,
the reserved space is inserted between the inner border edge
and the outer padding edge.
For the purpose of the <a spec=css-backgrounds>background positioning area</a> and <a spec=css-backgrounds>background painting area</a> however,
this reserved space is considered to be part of the [=padding box=].
<div class="example">
In the following document fragment,
both an absolutely-positioned element
and a background image
are positioned to the top right of the box.
<xmp highlight=html>
<style>
article {
background: top right no-repeat url(circle.png);
position: relative;
overflow: auto; }
aside { position: absolute; top: 0; right: 0; }
</style>
<article>
<aside>×</aside>
</article>
</xmp>
If no scrollbars are present on <code><article></code>,
they will both coincide in the top right padding edge corner.
However, if scrollbars are present
then <code><aside></code> will be completely visible,
on the right padding-box edge adjacent to the scrollbars;
whereas the background image will be tucked underneath the scrollbars,
in the top right corner of the scrollbar-extended <a spec=css-backgrounds>background positioning area</a>.
</div>
When the box is intrinsically sized,
this reserved space is added to the size of its contents.
It is otherwise subtracted from space alotted to the [=content area=].
To the extent that the presence of scrollbars can affect sizing,
UAs must start with the assumption that no scrollbars are needed,
and recalculate sizes if it turns out they are.
<div class="example">
In the following document fragment,
the outer <code><article></code> has ''height: auto'', but ''max-height: 5em''.
The inner <code><section></code> has large margins and would normally just fit:
<xmp highlight=html>
...
article { overflow: auto; max-height: 5em; width: max-content; }
section { margin: 2em; line-height: 1 }
...
<article>
<section>
This section has big margins.
</section>
</article>
</xmp>
If we assumed that <code><article></code> needed scrollbars,
then the height of <code><section></code>,
including the single line of text and twice 2em of margins,
adds up to 5em plus a scrollbar.
Since that is greater than 5em, the maximum allowed height,
it seems we made the right assumption and d1 indeed needs scrollbars.
However, we should have started by assuming that no scrollbars are needed.
In that case the content height of <code><article></code>
is exactly the maximum height of 5em,
proving that the assumption was correct
and <code><article></code> indeed should not have scrollbars.
</div>
<h3 id="scrolling-direction">
Scrolling Origin, Direction, and Restriction</h3>
The <dfn export>initial scroll position</dfn>,
that is, the initial position of
the box’s <a>scrollable overflow area</a>
with respect to its border box,
prior to any user or programmatic scrolling that changes it,
is dependent on the box’s <a>writing mode</a>,
and is by default the <a>block-start</a>/<a>inline-start</a> edge
of the box’s <a>padding edge</a>.
However, the 'align-content' and 'justify-content' properties [[!CSS-ALIGN-3]]
can be used to change this,
see [[css-align-3#overflow-scroll-position]].
Due to Web-compatibility constraints
(caused by authors exploiting legacy bugs to surreptitiously hide content from visual readers but not search engines and/or speech output),
UAs must clip the <a>scrollable overflow area</a>
of <a>scroll containers</a>
on the <a>block-start</a> and </a>inline-start</a> sides of the box
(thereby behaving as if they had no <a>scrollable overflow</a> on that side).
The viewport uses the <a>principal writing mode</a> for these calculations.
<h3 id="overflow-clip-margin">
Expanding Clipping Bounds: the 'overflow-clip-margin' property</h3>
<pre class=propdef>
Name: overflow-clip-margin
Value: <<visual-box>> || <<length [0,∞]>>
Initial: ''0px''
Inherited: no
Applies to: all elements
Computed value: the computed <<length>> and a <<visual-box>> keyword
Animation type: per computed value if the <<visual-box>> values match; otherwise discrete
</pre>
This property defines the <dfn export>overflow clip edge</dfn> of the box,
i.e. precisely <em>how far</em> outside its bounds
the box’s content is allowed to paint
before being clipped
by effects (such as ''overflow: clip'', above)
that are defined to clip to the box’s [=overflow clip edge=].
Values are defined as follows:
<dl dfn-type=value dfn-for=overflow-clip-margin>
: <dfn><<visual-box>></dfn>
::
Specifies the box edge to use as the [=overflow clip edge=] origin,
i.e. when the specified offset is zero.
Defaults to ''overflow-clip-margin/padding-box'' if omitted.
: <dfn><<length [0,∞]>></dfn>
::
The specified offset dictates
how much the [=overflow clip edge=] is expanded from
the specified box edge
Negative values are invalid.
Defaults to zero if omitted.
</dl>
The [=overflow clip edge=] is shaped in the corners
exactly the same way as an <a spec=css-backgrounds-3>outer box-shadow</a>
with a spread radius of the same cumulative offset
from the box’s [=border edge=].
See [[css-backgrounds-3#corner-shaping]]
and [[css-backgrounds-3#shadow-shape]],
noting in particular the formula for outsets beyond the [=border edge=].
Note: This property has no effect on boxes
with ''overflow: hidden'' or ''overflow: scroll'',
which are not defined to use the [=overflow clip edge=].
<h3 id="overflow-propagation">
Overflow Viewport Propagation</h3>
UAs must apply the 'overflow-*' values
set on the root element to the <a>viewport</a>
when the root element’s 'display' value is not ''display/none''.
However,
when the root element is an [[!HTML]] <{html}> element
(including <a href="https://html.spec.whatwg.org/multipage/introduction.html#html-vs-xhtml">XML syntax for HTML</a>)
whose 'overflow' value is ''overflow/visible'' (in both axes),
and that element has as a child
a <{body}> element whose 'display' value is also not ''display/none'',
user agents must instead apply the 'overflow-*' values
of the first such child element to the viewport.
The element from which the value is propagated must then have
a used 'overflow' value of ''overflow/visible''.
Note: Using [=containment=] disables
this special handling of the HTML <{body}> element.
See the [[CSS-CONTAIN-1#contain-property]] for details.
Note: ''overflow: hidden'' on the root element
might not clip everything outside the [=Initial Containing Block=]
if the ICB is smaller than the viewport,
which can happen on mobile.
If ''overflow/visible'' is applied to the viewport,
it must be interpreted as ''overflow/auto''.
If ''overflow/clip'' is applied to the viewport,
it must be interpreted as ''overflow/hidden''.
<h3 id=logical>
Flow Relative Properties: the 'overflow-block' and 'overflow-inline' properties</h3>
The following properties are processed according to the same processing model
as the Flow-Relative box model properties defined in [[css-logical-1#box]].
<pre class="propdef">
Name: overflow-inline, overflow-block
Value: <<'overflow'>>
Initial: visible
Applies to: Same as 'overflow-x' and 'overflow-y'
Inherited: no
Percentages: N/A
Computed value: Same as 'overflow-x' and 'overflow-y'
Animation type: discrete
</pre>
These properties correspond to the
'overflow-x' and 'overflow-y' properties.
The mapping depends on the element's 'writing-mode'.
<h3 id=smooth-scrolling caniuse=css-scroll-behavior>
Smooth Scrolling: The 'scroll-behavior' Property</h3>
<pre class=propdef>
Name: scroll-behavior
Value: auto | smooth
Initial: auto
Applies to: <a>scrolling boxes</a>
Inherited: no
Computed value: specified value
Animatable: no
Canonical Order: per grammar
</pre>
The 'scroll-behavior' property specifies the scrolling behavior for a <a>scrolling box</a>,
when scrolling happens due to navigation,
scrolling APIs [[!CSSOM-VIEW]],
or scroll snapping operations not initiated by the user [[!CSS-SCROLL-SNAP-1]].
Any other scrolls, e.g. those that are performed by the user, are not affected by this property.
When this property is specified on the root element, it applies to the <a>viewport</a> instead.
Note: The 'scroll-behavior' property of the HTML <{body}> element is <em>not</em> propagated to the viewport.
<dl dfn-type=value dfn-for=scroll-behavior>
<dt><dfn>auto</dfn>
<dd>
The <a>scrolling box</a> is scrolled in an <a spec=cssom-view lt="instant scroll">instant</a> fashion.
<dt><dfn>smooth</dfn>
<dd>
The <a>scrolling box</a> is scrolled in a <a spec=cssom-view lt="smooth scroll">smooth</a> fashion
using a user-agent-defined timing function over a user-agent-defined period of time.
User agents should follow platform conventions, if any. <!--fingerprint-->
</dl>
User agents may ignore this property. <!--fingerprint-->
<h2 id="scrollbar-gutter-property">
Reserving space for the scrollbar: the 'scrollbar-gutter' property</h2>
The space between the inner border edge and the outer padding edge
which user agents may reserve to display the scrollbar
is called the <dfn>scrollbar gutter</dfn>.
The 'scrollbar-gutter' property gives control to the author
over the presence of <a>scrollbar gutters</a>
separately from the ability to control the presence of scrollbars
provided by the 'overflow' property.
<pre class="propdef">
Name: scrollbar-gutter
Value: auto | stable && both-edges?
Initial: auto
Applies to: [=scroll containers=]
Inherited: no
Computed value: specified keyword(s)
Animation type: discrete
</pre>
This property affects the presence of <a>scrollbar gutters</a>
placed at the <a>inline start</a> edge or <a>inline end</a> edge of the box.
The presence of a <a>scrollbar gutter</a>
at the <a>block start</a> edge and <a>block end</a> edge of the box
cannot be controlled in this level,
and is determined the same way as the presence of <a>scrollbar gutters</a>
placed at the <a>inline start</a> edge or <a>inline end</a> edge of the box
when 'scrollbar-gutter' is ''scrollbar-gutter/auto''.
Scrollbars which by default are placed over the content box
and do not cause <a>scrollbar gutters</a> to be created
are called <dfn export>overlay scrollbars</dfn>.
Such scrollbars are usually partially transparent, revealing the content behind them if any.
Their appearance and size may vary
based on whether and how the user is interacting with them.
Scrollbars which are always placed in a <a>scrollbar gutter</a>,
consuming space when present,
are called <dfn export>classic scrollbars</dfn>.
Such scrollbars are usually opaque.
Whether <a>classic scrollbars</a> or <a>overlay scrollbars</a> are used,
the appearance and size of the scrollbar,
and whether scrollbars appear on the start or end edge of the box,
is UA defined.
Note: Which side a scrollbar appears on may depend on
operating system conventions,
[=bidirectionality=],
or other ergonomic considerations.
In the case of <a>classic scrollbars</a>,
the width of the <a>scrollbar gutter</a>,
if present (see below),
is the same as the width of the scrollbar.
In the case of <a>overlay scrollbars</a>,
no <a>scrollbar gutter</a> is present.
Note: There are known use cases that could be addressed
by enabling <a>scrollbar gutters</a>
for <a>overlay scrollbars</a>,
but no satisfactory design has been agreed to so far.
This could be addressed by future extensions of this property.
See [[css-overflow-4#sbg-ext]].
The values of this property have the following meaning:
<dl dfn-for="scrollbar-gutter" dfn-type=value>
<dt><dfn>auto</dfn>
<dd><a>Classic scrollbars</a> consume space by creating a <a>scrollbar gutter</a>
when 'overflow' is ''overflow/scroll'',
or when 'overflow' is ''overflow/auto'' and the box is overflowing.
<a>Overlay scrollbars</a> do not consume space.
<dt><dfn>stable</dfn>
<dd>The <a>scrollbar gutter</a> is present
for <a>classic scrollbars</a>
when 'overflow' is ''overflow/hidden'', ''overflow/scroll'', or ''overflow/auto'',
regardless of whether the box is actually overflowing.
<a>Overlay scrollbars</a> do not consume space.
Note: This does not change whether the scrollbar itself is visible,
only the presence of a gutter is affected.
<dt><dfn>both-edges</dfn>
<dd>If a <a>scrollbar gutter</a> would be present
on one of the inline start edge or the inline end edge of the box,
another <a>scrollbar gutter</a> must be present on the opposite edge as well.
</dl>
When the <a>scrollbar gutter</a> is present but the scrollbar is not,
or the scrollbar is transparent or otherwise does not fully obscure the <a>scrollbar gutter</a>,
the background of the <a>scrollbar gutter</a> must be painted as an extension of the padding.
As for the 'overflow' property,
when 'scrollbar-gutter' is set on the root element,
the user agent must apply it to the viewport instead,
and the used value on the root element itself is ''scrollbar-gutter: auto''.
However, unlike the 'overflow' property,
the user agent must not propagate 'scrollbar-gutter' from the HTML <{body}> element.
<div class=note>
Note: The following table summarizes the interaction of 'overflow' and 'scrollbar-gutter'
showing in which case space is reserved for a <a>classic scrollbar</a>'s <a>scrollbar gutter</a>.
<style>
.data.complex tbody th:last-of-type {
border-right: solid black 2px;
}
.data.complex tbody tr:nth-of-type(2n) td,
.data.complex tbody tr:nth-of-type(2n) th:not([rowspan]) {
background: rgba(127, 127, 127, 0.25);
}
</style>
<table class="data complex">
<caption>
Should a <a>classic scrollbar</a>'s <a>scrollbar gutter</a> be present?</caption>
<thead>
<tr>
<th>'overflow'
<th>'scrollbar-gutter'
<th>Overflowing
<th>Not overflowing
</thead>
<tr>
<th rowspan=2>''overflow/scroll''
<th>''scrollbar-gutter/auto''
<td>yes
<td>yes
<tr>
<th>''scrollbar-gutter/stable''
<td>yes
<td>yes
<tr>
<th rowspan=2>''overflow/auto''
<th>''scrollbar-gutter/auto''
<td>yes
<td>
<tr>
<th>''scrollbar-gutter/stable''
<td>yes
<td>yes
<tr>
<th rowspan=2>''overflow/hidden''
<th>''scrollbar-gutter/auto''
<td>
<td>
<tr>
<th>''scrollbar-gutter/stable''
<td>yes
<td>yes
<tr>
<th rowspan=2>''overflow/visible'', ''overflow/clip''
<th>''scrollbar-gutter/auto''
<td>
<td>
<tr>
<th>''scrollbar-gutter/stable''
<td>
<td>
</table>
</div>
<h2 id="auto-ellipsis">
Automatic Ellipses</h2>
<h3 id="text-overflow" caniuse="text-overflow">
Overflow Ellipsis: the 'text-overflow' property</h3>
<pre class="propdef">
Name: text-overflow
Value: clip | ellipsis
Initial: clip
Applies to: block containers
Inherited: no
Percentages: N/A
Computed value: specified keyword
Animation type: discrete
</pre>
This property specifies rendering when inline content overflows
its <a>end</a> line box edge
in the inline progression direction of its block container element ("the block")
that has 'overflow'
other than ''visible''.
Text can overflow for example when it is prevented from wrapping
(e.g. due to <code class="lang-css">white-space: nowrap</code>
or a single word is too long to fit).
Values have the following meanings:
<dl data-dfn-for="text-overflow" class="valuedef">
<dt><dfn id=overflow-clip dfn-type=value>clip</dfn><
<dd>
Clip inline content that overflows its block container element. Characters may be only partially rendered.
<dt><dfn id=overflow-ellipsis dfn-type=value>ellipsis</dfn>
<dd>
Render an ellipsis character (U+2026)
to represent clipped inline content.
Implementations may substitute a more language, script, or writing-mode appropriate
ellipsis character,
or three dots "..." if the ellipsis character is unavailable.
</dl>
The term "character" is used in this property definition
for better readability and means "grapheme cluster" [[!UAX29]]
for implementation purposes.
For the ellipsis value
implementations must hide characters and
<a href="https://www.w3.org/TR/CSS2/visuren.html#inline-boxes">
atomic inline-level elements</a>
at the <a>end</a> edge of the line as necessary to fit the ellipsis, and
place the ellipsis immediately adjacent
to the <a>end</a> edge of the remaining inline content.
The first character or
<a href="https://www.w3.org/TR/CSS2/visuren.html#inline-boxes">
atomic inline-level element</a>
on a line
must be clipped rather than ellipsed.
<div class=example>
<h4 id="bidi-ellipsis" class="no-num no-toc">Bidi ellipsis examples</h4>
These examples demonstrate which characters get hidden
to make room for the ellipsis in a bidi situation:
those visually at the end edge of the line.
Sample CSS:
<pre highlight=css>
div {
font-family: monospace;
white-space: pre;
overflow: hidden;
width: 9ch;
text-overflow: ellipsis;
}
</pre>
Sample HTML fragments, renderings, and your browser:
<table class="awesome-table data">
<thead><th>HTML<th>Reference rendering<th>Your Browser</thead>
<tr>
<td><code highlight=html><div>שלום 123456</div></code><td><div style="font-family:monospace">123456 ם…</div><td><div style="font-family: monospace; white-space: pre; overflow: hidden; width: 9ch; text-overflow: ellipsis">שלום 123456</div>
<tr><td><code highlight=html><div dir=rtl>שלום 123456</div></code><td><div style="font-family:monospace">…456 שלום</div><td><div style="font-family: monospace; white-space: pre; overflow: hidden; width: 9ch; text-overflow: ellipsis" dir=rtl>שלום 123456</div>
</table>
</div>
<h4 id="ellipsing-details" class="no-num no-toc">
ellipsing details</h4>
<ul>
<li>
Ellipsing only affects rendering and must not affect layout
nor dispatching of pointer events:
The UA should dispatch any pointer event on the ellipsis to the elided element,
as if 'text-overflow' had been ''text-overflow/none''.
<li>
The ellipsis is styled and baseline-aligned according to