fix(api): honor negative bottom margin, reject negative page margin#232
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A negative bottom margin on a composite was silently dropped by the vertical flow — its bottom-edge advance early-returned on a non-positive amount — while a negative page margin was accepted and silently overflowed the sheet. Composites now close their bottom edge through closeBottomSpace, which lets a negative bottom margin pull the following sibling up, symmetric with a negative top margin (which already offsets via placementTopY). The top-of-node reservation and the row/leaf paths are untouched, so existing layouts are byte-identical. DocumentSession now rejects a negative page margin with IllegalArgumentException pointing to bleed(); node margins still allow negatives for overlap tricks. Tests: NegativeMarginTest covers the page margin rejected via the builder and via the session setter, zero/positive accepted, and a negative bottom margin shifting the following section up by the margin. Full suite green, no visual baselines changed.
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Why
Two sides of the same gap in how the layout flow treats a negative margin:
What changed
closeBottomSpace. A positive bottom inset advances the flow as before; a negative one pulls the following sibling up — symmetric with a negative top margin, which already offsets viaplacementTopY. The top-of-node reservation, the inter-child spacing, and the row/leaf paths are untouched (rows/leaves already fold the bottom inset into their positive content height), so every existing layout is byte-identical.DocumentSession.margin(...)(and the builder'smargin(...)) now reject a negative page margin withIllegalArgumentException, pointing the caller at a node'sbleed(...)to reach the page edge. Node margins still allow negatives — they're a legitimate overlap/inset trick (e.g. a widget pulling up over the band above it).Lane: shared-engine (
LayoutCompiler) + canonical (DocumentSession). Purely additive at the signature level → japicmp-safe.Verification
./mvnw test -pl .— 1530 green, 0 visual baselines changed (the engine change is byte-identical for all existing content; the EPS boundary was checked to confirm no divergence for non-negative bottom insets).NegativeMarginTest: negative page margin rejected via the builder and via the session setter, zero/positive accepted, and a section with a negative bottom margin shifts the following section up by exactly that margin.Behavior note: callers that previously passed a negative page margin (and got silent overflow) now get a clear exception — intended, and called out in the CHANGELOG.