preventScroll does not work with dynamically created elements

[Elliot Nahman]

Safari 15 appears to have implemented the preventScroll option for Element.focus(). See the resolved webkit bug: https://bugs.webkit.org/show_bug.cgi?id=178583. 

While this feature works fine for elements already existing in the DOM, it does not work for dynamically added elements. 

Please see: https://codepen.io/nonmetalhail/pen/KKZzagd

[Simon Fraser (smfr)]

Thanks for the report. I can reproduce.

[Radar WebKit Bug Importer]

<rdar://problem/90502768>

[Simon Fraser (smfr)]

In the bad case we're scrolling via FrameSelection::updateAppearanceAfterLayoutOrStyleChange().

[Simon Fraser (smfr)]

Layout calls FrameSelection::setNeedsSelectionUpdate() which resets m_selectionRevealMode to SelectionRevealMode::Reveal here:

    if (innerTextSizeChanged && frame().selection().isFocusedAndActive() && document().focusedElement() == &inputElement()) {
        // The caps lock indicator was hidden or shown. If it is now visible then it may be occluding
        // the current selection (say, the caret was after the last character in the text field).
        // Schedule an update and reveal of the current selection.
        frame().selection().setNeedsSelectionUpdate(FrameSelection::RevealSelectionAfterUpdate::Forced);
    }

This is trying to ensure that text within the input is scrolled into view, but has the side effect of also triggering page scrolling.