Follow-up to PR #21690: these tests check that pressing Home/End
correctly updates the last focused menu-item index, so that a following
ArrowUp/ArrowDown press doesn't move focus to an unexpected menu-item.
Screen readers like NVDA fire synthetic click events on the <img> child of the button rather than on the
thumbnailImageContainer element itself, so the strict classList.contains check silently failed.
Update the styles and HTML to reflect the new views manager concept.
For now, nothing about split/merge functionality is implemented or visible.
The new styles for the outline, attachments, and layers will be added later.
The thumbnail view is now accessible with the keyboard.
This is a precursor to moving the call into a
worker thread to let us use `OffscreenCanvas`. The
current position wouldn't work since we make
transformations to the canvas object after the
getContext call, which isn't allowed for
OffscreenCanvas. Also it isn't allowed to clone or
`transferControlToOffscreen` the canvas after the
`getContext` call.
