MImage/MVideo pinned AttachmentBox to a fixed height computed for a 400px-wide
layout. The box width is responsive (maxWidth:100%) but the height was frozen,
so on a phone the box narrows below 400px while keeping desktop height ->
images crop (object-fit:cover) and videos letterbox (object-fit:contain).
Drive the box by `aspect-ratio: w/h` when intrinsic dimensions are known, so the
height tracks the responsive width. On desktop the box stays 400px wide, so the
aspect-ratio yields the identical height (algebraically 400*h/w =
scaleYDimension(w,400,h)) — pixel-identical. Falls back to the fixed height when
dimensions are unknown; the 48px floor and 600px cap are preserved.
Uses the same pattern already shipped in this codebase (TwitchThumbnailWrapper,
GalleryTile). Two review passes, one empirically measuring the rendered image in
Chromium: desktop unchanged, narrow widths keep correct aspect, no collapse.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
jaredandClaude Opus 4.8
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