On screen, placed documents (PDF, .ai and .vectorcraft) draw from
bitmaps made in the background. Each bitmap that became ready changed
a generation in the canvas key, so the canvas rendered the whole frame
again. The bitmaps share a 512 MiB budget, and making one dropped the
least recently drawn others, even those on screen: with more than that
on screen, every frame dropped a bitmap it drew and asked for it again,
without end. A bitmap that could not be made (the file can't be read
and there is no preview) also changed the generation and was asked for
again in every frame.
When a bitmap is ready, placed_document::made_between names its file,
and the canvas renders again only where the objects showing it are
(placed_area), as it does after an edit, when the frame shown was
rendered on the UI thread and that part is at most half the frame.
Art of a symbol, a graph design, a pattern or a brush that shows the
file still renders the whole frame.
Bitmaps drawn or made since a frame began are not dropped to make room
for the ones it asks for. They stay past the budget up to 2 GiB. A
bitmap that would take them past 2 GiB is not kept, and its file draws
exactly at that zoom. A bitmap that can't be made shows the
placeholder, does not change the generation, and is not asked for
again.
Docs: docs/target-app-parity.md.
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