Adjusting the top, bottom, left, or right page margin had no visible
effect until an unrelated setting (e.g. Show Header) was toggled.
The BooksGrid perf refactor (#4562) memoized the derived view/content
insets on the ViewSettings object identity. saveViewSettings mutates
ViewSettings in place (same reference), so the memo never recomputed on
a margin edit and the new margin never reached the paginator. Left and
right margins were always stale; top and bottom only refreshed when the
header/footer visibility (an effect dependency) changed, which is why
toggling the header appeared to apply a pending change.
Extract the inset derivation into useContentInsets and memoize by the
resolved numeric values instead of the object reference: identical
numbers across a page turn keep a stable reference (no re-render storm),
while a changed margin yields a new one that propagates to the renderer.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Huang Xin
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