Review of #2815 (CodeRabbit):
- A content stream whose bytes live in an external file (`/F`) draws
those, not its inline bytes, so it is never taken for a backdrop.
- The copy keeps the original stream dictionary, unknown entries
included, less `Length` and `DL` (Flate re-encodes it); it had started
from an empty dictionary.
- Every `/Contents` slot that lists the original stream gets the copy: a
page listing the same stream twice painted the white fill again over
the background.
- The Background dialog starts with replacing on each time it opens.
Update restores backdrops before re-applying, so a replaced page shows
no fill and no checkbox, and a choice left off from an earlier dialog
hid the background again unseen.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>