A perfect implementation of this would probably leave the filter/header
content visible, but that requires some re-thinking and restructuring to
how the content is displayed in these overlays (ie. the header
component shouldn't be inside the `ScrollContainer` as it is fixed).
Supersedes and closes#10774.
Closes#933.
Addresses most pieces of #7417.
I tried restructuring the hierarchy to avoid needing this added property
(moving the hover layer out of the border container) but this leads to
some subpixel leakage outside the borders which looks even worse.
Closes#6915.