Previously, has_scrollable_overflow was a purely geometric check, true
whenever content extended beyond the padding box regardless of the
overflow property. This caused unnecessary scroll frame allocation for
boxes with `overflow:visible`.
Per CSS Overflow 3, scrollable overflow is only defined for scroll
containers (overflow: auto/hidden/scroll). Gate the flag on
`is_scroll_container()` so that only actual scroll containers get scroll
frames assigned.
Compute inline-block baselines by traversing into nested block children
to find the last in-flow line box, using correct offsets relative to the
margin box edge.
Also ensure inline-flex and inline-grid containers always derive their
baseline from content (per CSS Align), and add special handling for
<input> elements which have `overflow: clip` in the UA stylesheet but
should still align adjacent text with their internal content.
When we compute style for elements inside a UA-internal shadow tree that
represent a pseudo-element (e.g ::placeholder), we actually run the
StyleComputer machinery for (host element :: pseudo-element).
While that lets us match the correct selectors, it was incorrectly
applying CSS inheritance, since we'd also then inherit from whatever was
above the host element in the tree.
This patch fixes the issue by introducing an inheritance override in
AbstractElement and then using that to force inheritance from whatever
is actually directly above in the DOM for these elements instead of
jumping all the way up past the host.
This fixes an issue where `text-align: center` on input type=text
elements would render the main text centered but placeholder text was
still left-aligned.
