Use visual viewport instead of initial viewport in screenshot algorithm (#1951)

Updates the screenshot algorithm to stay aligned with WebDriver BiDi.

Fixes #1947

Author: Sameem-baba

index.html

@@ -10926,12 +10926,12 @@ <h2>Screen capture</h2>
 
 <p>Screenshots are a mechanism for providing
  additional visual diagnostic information.
- They work by dumping a snapshot of the <a>initial viewport</a>&apos;s
+ They work by dumping a snapshot of the <a>visual viewport</a>&apos;s
  framebuffer as a lossless PNG image.
  It is returned to the <a>local end</a> as a Base64 encoded string.
 
 <p>WebDriver provides the <a>Take Screenshot</a> <a>command</a>
- to capture the <a>top-level browsing context</a>&apos;s <a>initial viewport</a>,
+ to capture the <a>top-level browsing context</a>&apos;s <a>visual viewport</a>,
  and a <a>command</a> <a>Take Element Screenshot</a>
  for doing the same with the visible region of an <a>element</a>&apos;s
  <a>bounding rectangle</a> after it has been <a>scrolled into view</a>.
@@ -10940,15 +10940,15 @@ <h2>Screen capture</h2>
  given a <a>rectangle</a>:
 
 <ol>
- <li><p>If either the <a>initial viewport</a>&apos;s width or height
+ <li><p>If either the <a>visual viewport</a>&apos;s width or height
   is 0 <a>CSS pixels</a>,
   return <a>error</a> with <a>error code</a> <a>unable to capture screen</a>.
 
- <li><p>Let <var>paint width</var> be the <a>initial viewport</a>&apos;s width –
+ <li><p>Let <var>paint width</var> be the <a>visual viewport</a>&apos;s width –
   <a>min</a>(<a>rectangle x coordinate</a>,
   <a>rectangle x coordinate</a> + <a>rectangle width dimension</a>).
 
- <li><p>Let <var>paint height</var> be the <a>initial viewport</a>&apos;s height –
+ <li><p>Let <var>paint height</var> be the <a>visual viewport</a>&apos;s height –
   <a>min</a>(<a>rectangle y coordinate</a>,
   <a>rectangle y coordinate</a> + <a>rectangle height dimension</a>).
 
@@ -11973,6 +11973,7 @@ <h2>Index</h2>
    <!-- ScrollIntoViewOptions --> <li><dfn><a href=https://drafts.csswg.org/cssom-view/#dictdef-scrollintoviewoptions><code>ScrollIntoViewOptions</code></a></dfn>
    <!-- ScrollIntoViewOptions block --> <li><dfn><a href=https://drafts.csswg.org/cssom-view/#dom-scrollintoviewoptions-block>Logical scroll position "<code>block</code>"</a></dfn>
    <!-- ScrollIntoViewOptions inline --> <li><dfn><a href=https://drafts.csswg.org/cssom-view/#dom-scrollintoviewoptions-inline>Logical scroll position "<code>inline</code>"</a></dfn>
+   <!-- visual viewport --> <li><dfn><a href="https://drafts.csswg.org/cssom-view/#visual-viewport">visual viewport</a></dfn>
   </ul>
 
  <dd>The following terms are defined in  [[mediaqueries-4]]: