Why: .main { display: flex } overrides the UA [hidden] rule, so previewExpanded left the conversation column in the row and squeezed the preview.
Evidence: vitest test/previewfit.test.ts and test/design-guard.test.ts passed (10 tests). Headless Chromium at 1512x900: expanded .main is display:none and the preview slot is 1184px with the page centered (center delta 0); at 420px and 640px the wrapper stays column-width and the page stays left-aligned.
Impact: expanded preview fills the main row; docx pages center only when the column is wider than the page. Narrow-column fit scaling is unchanged. previewfit still returns 1 when available width is <= 0.
Refs: App.tsx hidden={previewExpanded}; styles.css .main and .docx-wrapper
Co-authored-by: Cursor <cursoragent@cursor.com>
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