In auto-spread mode a portrait viewport shows a single page of the
two-page spread, but the fixed-layout renderer kept the spread-centering
one-sided inline margin on the shown page. With no partner page to meet
at the spine, that margin stranded the lone page in one half of the
viewport whenever it was narrower than the viewport (any zoom below
100%, or a page whose fit-scaled width is less than the viewport width),
which also pushed the page over a page-turn tap zone so every tap turned
the page instead of opening the menu.
Bump the foliate-js submodule (readest/foliate-js#50) to center the lone
portrait page and add a regression test for computeSpreadInlineMargins.
Fixes#4984
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Huang Xin
3ce5a5c8e3