[css-text] Clarifications on "hanging"

Separate the notion of hanging in general, from it's specific
application to hanging-punctuation, and provide reusable anchor
terminology that distinguishes unconditional hanging from conditional hanging.

Also, be explicit about the effects on intrinsic sizes.

Closes w3c#3784

Note: there are open issues and pending edits on how preserved
white-space at the end of a line is supposed to hang. This edit does not
solve these, and keeps the current behavior as it is. By making the
definitions of hanging reusable, it is a useful step towards solving
these as well.

Author: frivoal

css-text-3/Overview.bs

@@ -802,7 +802,7 @@ Text Processing</h3>
           word-break/word-break-break-all-017.html
         </wpt>
 
-      Such preserved spaces take up space and do not hang,
+      Such preserved spaces take up space and do not [=hang=],
       and thus affect the box's intrinsic sizes
       ([=min-content size=] and [=max-content size=]).
 
@@ -843,11 +843,9 @@ Text Processing</h3>
     In general, <a>preserved white space</a> is measured when calculating
     the intrinsic sizes ([=min-content size=] and [=max-content size=]) of text;
     however <a>preserved white space</a>
-    that would be allowed to <a>hang</a> at the end of the line
+    that would <a>hang</a> when at the end of the line
     (i.e. that preserved by ''pre-wrap'')
-    is excluded--
-    that is, considered to <a>hang</a>--
-    when calculating <a>min-content sizes</a> and any sizes derived thereof.
+    is excluded.
 
   <p>The following informative table summarizes the behavior of various
     'white-space' values:</p>
@@ -1535,8 +1533,7 @@ Text Processing</h3>
         at the end of a line:
         <ul>
           <li>If 'white-space' is set to ''pre-wrap'',
-             the UA must <a>hang</a> this sequence
-             if it does not otherwise fit prior to justification.
+             the UA must [=conditionally hang=] this sequence.
              It may also visually collapse the character advance widths
              of any that would otherwise overflow.
 
@@ -1572,10 +1569,10 @@ Text Processing</h3>
                white-space/white-space-intrinsic-size-004.html
              </wpt>
 
-             Note: Hanging the white space rather than collapsing it
+             Note: [=Hanging=] the white space rather than collapsing it
              allows users to see the space when selecting or editing text.
           <li>If 'white-space' is set to ''break-spaces'',
-             hanging or collapsing the advance width of spaces
+             [=hanging=] or collapsing the advance width of spaces
              at the end of the line is not allowed;
              those that overflow must <a>wrap</a> to the next line.
 
@@ -4186,8 +4183,60 @@ Cursive Scripts</h4>
       For this reason, it is often wise to specify 'text-indent: 0' on
       elements that are specified 'display: inline-block'.</p>
 
-  <h3 id="hanging-punctuation-property" caniuse="css-hanging-punctuation" oldids="hanging-punctuation">
-    Hanging Punctuation: the 'hanging-punctuation' property</h3>
+  <h3 id="hanging">
+    Hanging Glyphs</h3>
+
+    <p>When a glyph at the start or end edge of a line <dfn lt="hang">hangs</dfn>,
+      it is not considered
+      when measuring the line's contents for fit, alignment, or justification.
+      Depending on the line's alignment/justification, this can
+      result in the mark being placed outside the line box.
+      The [=hanging=] glyph is also not taken into account
+      when computing intrinsic sizes (min-content size and max-content size),
+      and any sizes derived thereof.
+      (The interaction of this measurement and kerning is currently UA-defined;
+      the CSSWG <a href="https://github.com/w3c/csswg-drafts/issues/2397">welcomes advice</a> on this point.)
+
+    <p>A <a lt=hang>hanging</a> glyph
+      is still enclosed inside its parent inline box,
+      is still counted as part of the <a>scrollable overflow region</a> [[!CSS-OVERFLOW-3]],
+      and still participates in text justification:
+      its character advance is just not measured when determining
+      how much content fits on the line,
+      how much the line's contents need to be expanded or compressed for justification,
+      or how to position the content within the line box for text alignment.
+      Effectively, the <a>hanging</a> glyph character advance
+      is re-interpreted as an additional negative margin
+      on the affected edge of its parent <a>inline box</a>;
+      the line is otherwise laid out as usual.
+
+      <wpt>
+        hanging-punctuation/hanging-scrollable-001.html
+      </wpt>
+
+    <p>In some cases, a glyph at the end of a line
+      can <dfn>conditionally hang</dfn>:
+      it [=hangs=] only if it does not otherwise fit in the line prior to justification.
+      It is not considered when measuring the line’s contents for fit.
+      However, any part of it that does not fit
+      is considered to [=hang=].
+      Glyphs that [=conditionally hang=] are not taken into account
+      when computing [=min-content sizes=]
+      and any sizes derived thereof,
+      but they are taken into account for [=max-content sizes=]
+      and any sizes derived thereof.
+
+    <p>Non-zero inline-axis borders or padding between
+      a <a>hang</a>able glyph and the edge of the line prevent the glyph from hanging.
+      For example, a period at the end of an inline box with end padding
+      does not <a>hang</a> at the end edge of a line.
+
+    <p>Multiple adjacent glyphs can hang together,
+      however there may be limits on how many are allowed to hang
+      (e.g. at most one punctuation character may <a>hang</a> at each edge of the line).
+
+  <h4 id="hanging-punctuation-property" caniuse="css-hanging-punctuation" oldids="hanging-punctuation">
+    Hanging Punctuation: the 'hanging-punctuation' property</h4>
 
     <pre class="propdef">
     Name: hanging-punctuation
@@ -4213,18 +4262,11 @@ Cursive Scripts</h4>
     <p class="note">Note: If there is not sufficient padding on the
       block container, 'hanging-punctuation' can trigger overflow.</p>
 
-    <p>When a glyph <dfn lt="hang">hangs</dfn>, it is not considered
-      when measuring the line's contents for fit, alignment, or justification.
-      Depending on the line's alignment/justification, this can
-      result in the mark being placed outside the line box.
-      (The interaction of this measurement and kerning is currently UA-defined;
-      the CSSWG <a href="https://github.com/w3c/csswg-drafts/issues/2397">welcomes advice</a> on this point.)
-
     <p>Values have the following meanings:</p>
 
     <dl dfn-for=hanging-punctuation dfn-type=value>
       <dt><dfn>none</dfn></dt>
-        <dd>No character <a>hangs</a>.</dd>
+        <dd>No punctuation character is made to <a>hang</a>.</dd>
       <dt><dfn>first</dfn></dt>
         <dd>An opening bracket or quote at the start of the
           <a href="https://www.w3.org/TR/CSS2/selector.html#first-line-pseudo">first
@@ -4252,37 +4294,15 @@ Cursive Scripts</h4>
         </wpt>
         </dd>
       <dt><dfn>allow-end</dfn></dt>
-        <dd>A <a>stop or comma</a> at the end of a line <a>hangs</a> if it
-          does not otherwise fit prior to justification.
+        <dd>A <a>stop or comma</a> at the end of a line <a>conditionally hangs</a>.
 
           <wpt>
             hanging-punctuation/hanging-punctuation-allow-end-001.xht
           </wpt>
         </dd>
     </dl>
 
-    <p>Non-zero inline-axis borders or padding between
-      a <a>hang</a>able mark and the edge of the line prevent the mark from hanging.
-      For example, a period at the end of an inline box with end padding
-      does not <a>hang</a> at the end edge of a line.
-      At most one punctuation character may <a>hang</a> at each edge of the line.
-
-    <p>A <a lt=hang>hanging</a> mark
-      is still enclosed inside its parent inline box,
-      is still counted as part of the <a>scrollable overflow region</a> [[!CSS-OVERFLOW-3]],
-      and still participates in text justification:
-      its character advance is just not measured when determining
-      how much content fits on the line,
-      how much the line's contents need to be expanded or compressed for justification,
-      or how to position the content within the line box for text alignment.
-      Effectively, the <a>hanging</a> glyph character advance
-      is re-interpreted as an additional negative margin
-      on the affected edge of its parent <a>inline box</a>;
-      the line is otherwise laid out as usual.
-
-      <wpt>
-        hanging-punctuation/hanging-scrollable-001.html
-      </wpt>
+    <p>At most one punctuation character may <a>hang</a> at each edge of the line.
 
     <p><dfn lt="stop or comma">Stops and commas</dfn> allowed to <a>hang</a> include:
     <table class="data">