fix(macos): stop zero-width measurements from poisoning chat cell caches#26316
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During the first LazyVStack layout pass, MessageListLayoutMetrics deliberately reports bubbleMaxWidth = 0 until GeometryReader resolves the chat column width. That zero flowed unchecked into MarkdownSegmentView.effectiveMaxWidth and VSelectableTextView.measureSize(maxWidth: 0), where ensureLayout returned a usedRect with height 0. Before the recent sizeThatFits-caching commit (#26170), the degenerate result self-healed on the next body pass; with the new caches, (0,0) was persisted at the attributedString + 0 + 4 key and the MarkdownSegmentView NSCache entry keyed on effectiveMaxWidth = 0, collapsing the cell and stacking every visible assistant message body at the same y. Two small, orthogonal guards: - VSelectableTextView.measureSize refuses maxWidth <= 0 or empty strings with an early .zero return (no cache write), and only writes non-zero heights into measurementSizeCache. - MarkdownSegmentView.resolveSelectableRunMeasurementResult skips the measuredTextCache insertion when size.height == 0. Both keep the perf wins from #26170 and #26242 (no revert). Adds a regression test that measures at maxContentWidth: 0 and asserts no cache poisoning. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Summary
MessageListLayoutMetrics(clients/macos/vellum-assistant/Features/Chat/MessageListLayoutMetrics.swift:14-32) reportsbubbleMaxWidth = 0untilGeometryReaderresolves the chat column width. That zero cascades throughMarkdownSegmentView.effectiveMaxWidth(line 404 uses??which only fires onnil, not on0) intoVSelectableTextView.measureSize(maxWidth: 0), whereensureLayoutreturns ausedRectwithheight == 0..frame(width: 0, height: 0)applied bySelectableRunViewcollapses the cell — all visible assistant messages stack at the same y.VSelectableTextView.measurementSizeCacheandMarkdownSegmentView.measuredTextCache) persisted the(0,0)result at the 0-width cache key, so the cell stayed collapsed.Changes
VSelectableTextView.measureSizeguardsmaxWidth > 0 && attributedString.length > 0and returns.zerowithout writing the cache. Also only persists non-zero heights inmeasurementSizeCache.MarkdownSegmentView.resolveSelectableRunMeasurementResultaddssize.height > 0 &&to themeasuredTextCacheinsertion guard.testZeroWidthReturnsZeroSizeWithoutPoisoningCachesregression test.Test plan
clients/macos/build.sh), open a long multi-message conversation, confirm each assistant body renders at a distinct y with no overlap.xcodebuild testforMarkdownSegmentViewTests— new regression test passes, existing tests unaffected.🤖 Generated with Claude Code