The displayport clip that is applied to sticky items from the enclosing
scrollframe can cause sticky items to checkerboard even when they might
reasonably be left visible. This is because the displayport clip moves as
the enclosing scrollframe is scrolled, but sticky items may remain fixed
during such scrolling. The displayport clip can therefore clip out sticky
items even if they are "stuck" and should be user-visible.
This patch sets a flag to identify when a sticky item is being clipped by
the displayport clip, and ensures that it doesn't actually get clipped. In
the case where other clips are being applied to the sticky item, we leave the
clips unaffected. This allows for other enclosing elements to clip the sticky
item as before.
Differential Revision: https://phabricator.services.mozilla.com/D68582
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extra : moz-landing-system : lando
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