[cssom-view-1] Add PlainTextRange geometry APIs (getClientRects/getBoundingClientRects)

cssom-view-1/Overview.bs

@@ -56,6 +56,10 @@ spec:css-borders-4; type:property; text:border-top-width
 spec:css-borders-4; type:property; text:border-left-width
 spec:cssom-view-1; type:dictionary; text:MouseEventInit
 spec:cssom-view-1; type:dfn; for:MediaQueryList; text:media
+spec:dom; type:interface; text:PlainTextRange
+spec:dom; type:method; for:PlainTextRange; text:getClientRects()
+spec:dom; type:method; for:PlainTextRange; text:getBoundingClientRect()
+spec:dom; type:dfn; for:PlainTextRange; text:plain text range string
 </pre>
 
 <style>
@@ -2044,6 +2048,54 @@ Note: The {{DOMRect}} object returned by {{Range/getBoundingClientRect()}} is no
     range-bounding-client-rect-with-display-contents.html
 </wpt>
 
+<h2 id=extensions-to-the-plaintextrange-interface>
+  Extensions to the {{PlainTextRange}} Interface
+</h2>
+
+This section defines {{PlainTextRange/getClientRects()}} and
+{{PlainTextRange/getBoundingClientRect()}} for {{PlainTextRange}}. These mirror {{Range}}’s
+{{Range/getClientRects()}} and {{Range/getBoundingClientRect()}}, but operate over the portion of
+the host’s <a for=PlainTextRange>plain text range string</a> selected by the range.
+
+<div algorithm="getClientRects() for PlainTextRange">
+  The steps for {{PlainTextRange/getClientRects()}} are the same as {{Range/getClientRects()}},
+  except that the rectangles are computed relative to the rendered text of the range’s
+  <a for=PlainTextRange>host</a>.
+  <ol>
+    <li>If <a for=PlainTextRange>host</a> is null, not <a>connected</a>, or its computed
+        <code>display</code> is <code>none</code>, return an empty {{DOMRectList}}.</li>
+    <li>Update style and layout for <a for=PlainTextRange>host</a>.</li>
+    <li>Let <var>string</var> be <a for=PlainTextRange>host</a>’s
+        <a for=PlainTextRange>plain text range string</a>, and let <var>len</var> be
+        <var>string</var>’s <a for=string>length</a>.</li>
+    <li>Let <var>start</var> be min(<a>this</a>.{{AbstractRange/startOffset}}, <var>len</var>) and let
+        <var>end</var> be min(<a>this</a>.{{AbstractRange/endOffset}}, <var>len</var>).</li>
+    <li>If <var>start</var> = <var>end</var>, return an empty {{DOMRectList}}.</li>
+    <li>Let <var>textNode</var> be the single {{Text}} node representing the rendered value of
+        <a for=PlainTextRange>host</a>. If none exists, return an empty {{DOMRectList}}.</li>
+    <li>Create a {{Range}} <var>r</var> whose start is (<var>textNode</var>, <var>start</var>) and
+        whose end is (<var>textNode</var>, <var>end</var>).</li>
+    <li>Return <var>r</var>.{{Range/getClientRects()}}.</li>
+  </ol>
+</div>
+
+<div algorithm="getBoundingClientRect() for PlainTextRange">
+  The steps for {{PlainTextRange/getBoundingClientRect()}} are the same as
+  {{Range/getBoundingClientRect()}}, except that the union of rectangles is computed relative to
+  the rendered text of the range’s <a for=PlainTextRange>host</a>.
+  <ol>
+    <li>Let <var>rects</var> be the result of {{PlainTextRange/getClientRects()}}.</li>
+    <li>If <var>rects</var> is empty, return a {{DOMRect}} whose {{DOMRect/x}}, {{DOMRect/y}},
+        {{DOMRect/width}}, and {{DOMRect/height}} are zero.</li>
+    <li>Otherwise, return a {{DOMRect}} representing the smallest rectangle that encloses all
+        rectangles in <var>rects</var>.</li>
+  </ol>
+</div>
+
+<wpt>
+  PlainTextRange-geometry.html
+</wpt>
+
 <h2 id=extensions-to-the-mouseevent-interface>Extensions to the {{MouseEvent}} Interface</h2>
 
 Issue: The object IDL fragment redefines some members. Can we resolve this somehow?