[cssom-view] Specify partial grapheme clusters for range.getClientRects()

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=29562
Ref. https://lists.w3.org/Archives/Public/www-style/2016Apr/0115.html

Author: zcorpan

cssom-view/Overview.bs

@@ -1424,7 +1424,15 @@ static {{DOMRect}} objects in content order that matches the
 following constraints:
 
 * For each element selected by the range, whose parent is not selected by the range, include the border areas returned by invoking {{Element/getClientRects()}} on the element.
-* For each {{Text}} node selected or partially selected by the range (including when the boundary-points are identical), include a {{DOMRect}} object (for the part that is selected, not the whole line box). The bounds of these {{DOMRect}} objects are computed using font metrics; thus, for horizontal writing, the vertical dimension of each box is determined by the font ascent and descent, and the horizontal dimension by the text advance width. The <a>transforms</a> that apply to the ancestors are applied.
+* For each {{Text}} node selected or partially selected by the range (including when the
+    boundary-points are identical), include a {{DOMRect}} object (for the part that is selected, not
+    the whole line box). The bounds of these {{DOMRect}} objects are computed using font metrics;
+    thus, for horizontal writing, the vertical dimension of each box is determined by the font
+    ascent and descent, and the horizontal dimension by the text advance width. If the range covers
+    a partial <a spec=css-text>typographic character unit</a> (e.g. half a surrogate pair or part of
+    a grapheme cluster), the full <a spec=css-text>typographic character unit</a> must be included
+    for the purpose of computing the bounds of the relevant {{DOMRect}}. [[!CSS-TEXT-3]] The
+    <a>transforms</a> that apply to the ancestors are applied.
 
 The <dfn method for=Range>getBoundingClientRect()</dfn> method, when invoked, must return the result of the following
 algorithm: