[cssom-view-1] Define OpaqueRange geometry APIs (getClientRects/getBoundingClientRect)

cssom-view-1/Overview.bs

@@ -58,6 +58,8 @@ spec:cssom-view-1; type:dfn; for:MediaQueryList; text:media
 spec:webdriver-bidi; type:dfn;
     text:WebDriver BiDi emulated available screen area
     text:WebDriver BiDi emulated total screen area
+spec:dom; type:interface; text:OpaqueRange
+spec:dom; type:dfn; text:opaque range string
 </pre>
 
 <style>
@@ -2081,6 +2083,49 @@ Note: The {{DOMRect}} object returned by {{Range/getBoundingClientRect()}} is no
     range-bounding-client-rect-with-display-contents.html
 </wpt>
 
+<h2 id=extensions-to-the-opaquerange-interface>Extensions to the {{OpaqueRange}} Interface</h2>
+
+This section defines {{OpaqueRange/getClientRects()}} and {{OpaqueRange/getBoundingClientRect()}} for
+{{OpaqueRange}}. These mirror {{Range}}'s {{Range/getClientRects()}} and
+{{Range/getBoundingClientRect()}}, but operate over the selected portion of the opaque range string
+(as defined by the relevant specification for the element the range was created from), as rendered.
+
+The <dfn method for=OpaqueRange>getClientRects()</dfn> method, when invoked, must run these steps:
+
+1. Let <var>element</var> be the element the range was created from.
+1. If the user agent cannot compute geometry for the range (e.g. because <var>element</var> is not
+    <a>connected</a> or its computed <code>display</code> is <code>none</code>), return an empty
+    {{DOMRectList}}.
+1. Update style and layout for <var>element</var>.
+1. Let <var>start</var> be <a>this</a>'s <a for=OpaqueRange>start offset</a>, and let <var>end</var>
+    be <a>this</a>'s <a for=OpaqueRange>end offset</a>.
+1. If <var>start</var> equals <var>end</var>, then:
+    1. If no visible caret would be rendered for <var>element</var> at offset <var>start</var>,
+        return an empty {{DOMRectList}}.
+    1. Return a {{DOMRectList}} containing a single {{DOMRect}} describing the caret rectangle for
+        <var>element</var> at offset <var>start</var>.
+1. Return a {{DOMRectList}} containing the client rectangles that enclose the rendered text
+    corresponding to the substring of <var>element</var>'s <a>opaque range string</a> starting at
+    <var>start</var> (inclusive) and ending at <var>end</var> (exclusive).
+
+The <dfn method for=OpaqueRange>getBoundingClientRect()</dfn> method, when invoked, must return the
+result of the following algorithm:
+
+1. Let <var>list</var> be the result of invoking {{OpaqueRange/getClientRects()}} on the same range
+    this method was invoked on.
+1. If <var>list</var> is empty return a {{DOMRect}} object whose {{DOMRect/x}}, {{DOMRect/y}},
+    {{DOMRect/width}} and {{DOMRect/height}} members are zero.
+1. If all rectangles in <var>list</var> have zero width or height, return the first rectangle in
+    <var>list</var>.
+1. Otherwise, return a {{DOMRect}} object describing the smallest rectangle that includes all of the
+    rectangles in <var>list</var> of which the height or width is not zero.
+
+<wpt>
+    OpaqueRange-geometry-basic.html
+    OpaqueRange-geometry-complexity-and-visibility.html
+    OpaqueRange-geometry-multiline-and-mutations.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?