While the right sidebar is collapsed, its docked hosts are translated past
the frame's right edge and widen the frame's scrollable overflow. Outside
macOS, PageUp in a text field moves the caret and reveals it against the
start edge of every scrollable ancestor, so the `overflow: hidden` frame
scrolled sideways and the whole UI shifted left.
Clip the panel's overflow. `overflow: clip` does not make the panel a scroll
container and does not clip the fixed-position floating panels, whose
containing block is the viewport; the DockLayout ancestor contract now says so.
Fixes#6341
imccyu
8953554b5e