When zooming, webrender overrides the raster space used to render text, so that
we do not expensively rerasterize the glyphs for every fractional change in zoom
level. Previously we chose to do this when any ancestor of the picture's spatial
node was being zoomed. This worked on most pages, because the scroll frame which
is used as the main picture caching slice is a descendent of the zooming
reference frame.
However, on pages without a scrollable frame, or for fixed-position content, the
picture's spatial node will not be a descendent of the zooming reference
frame. This meant that we did not detect that we were zooming and rendered the
text in screen raster space rather than the overridden local space, leading to
poor zooming performance.
To fix this, check whether the primitive's spatial node (rather than the
picture's) is a descendent of the zooming frame.
Differential Revision: https://phabricator.services.mozilla.com/D88474
b1d6fddf77