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This implements `document.scrollingElement`
(https://drafts.csswg.org/cssom-view/#dom-document-scrollingelement).
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- [x] These changes fix#35700
- [x] There are tests for these changes
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Signed-off-by: JimmyDdotEXE <50691404+JimmyDdotEXE@users.noreply.github.com>
Previously, the "scroll to fragment" operation could scroll past the end
of the screen, because the scroll position was not clamped to viewport
boundaries. Correct this by using the `Window::scroll()` method which
handles this case.
In addition, ensure that `Window`'s `current_viewport` member is
initialized properly when it is created.
Most tests were only being run for layout-2013, not for layout-2020.
This wasn't great since layout-2020 is now the default.
So this patch unifies the lists of included tests for both layouts.
For layout-2013 this implies adding css/css-content/, css/css-logical/
and css/css-masking/clip/.
For layout-2020 this implies adding several additional css tests, and
also tests like dom/, js/, html/, etc.
