On screen, a placed document is drawn from a bitmap made in the
background, and that bitmap was rendered with the default options. An
image inside the placed file whose pixels cannot be decoded therefore
drew nothing in the bitmap, even though the canvas marks such images
with a crossed box. The bitmap now follows the frame's
mark_undecoded_images setting, the bitmap cache keys on it, and a
bitmap standing in for one still being made has the same setting.
set_loader keeps the first registration, so a test registering blue_page first left the gate
out. Every test in the file now registers gated_blue_page (blue_page while the gate is open).
a_ready_bitmap_draws_again_where_its_file_shows assumed the first frame shows placeholders,
but the background worker can make the tiny test bitmaps between scheduling and the frame's
second lookup, so the frame already showed them ("the bitmaps show" failed on FreeBSD CI).
The test's loader now waits until that frame is drawn.
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