fix(engine): correct PageBackgroundFill y-coordinate for partial-height bands#75
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…ht bands A page-background fill with heightRatio < 1.0 was painted from the page bottom upward instead of from the documented yRatio top edge, so a band with yRatio=0 rendered at the bottom of the page. PlacedFragment.y is the PDF-native bottom-left origin (y grows up — see PdfShapeFragmentRenderHandler), so the top-down ratios now convert via y = (1 - yRatio - heightRatio) * pageHeight. Full-page and full-height column fills resolve to y=0, unchanged, so existing fills and visual baselines are unaffected. Adds top/bottom/mid-band regression tests — partial-height placement was previously untested because every factory helper uses heightRatio=1, where top and bottom are indistinguishable.
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Summary
Fixes the
PageBackgroundFilly-coordinate so a partial-height band lands where the API documents.The bug
PlacedFragment.yis the PDF-native bottom-left origin (y grows up;PdfShapeFragmentRenderHandlercallsaddRect(x, y, w, h)with(x, y)as the bottom-left).DocumentPageBackgrounds.apply()sety = yRatio * pageHeight, so a band withyRatio = 0— documented as the top edge — rendered at the bottom. Latent until now because every factory helper usesheightRatio = 1, where top and bottom are indistinguishable.The fix
y = (1 - yRatio - heightRatio) * pageHeight. Full-page and full-height column fills resolve toy = 0— unchanged, so existing fills and visual baselines are unaffected.Test plan
PageBackgroundTest13/13 greenmvnw clean verify -pl .— 997 tests, 0 failures (no visual baseline shifted)Part of the v1.6.5 engine work (audit §4, variant A). PR-7 adds
topBand/bottomBand/bandfactory helpers on top of this corrected coordinate model.