Stretch multi-row grid tracks with an auto maximum #106

Problem

A definite-height multi-row grid using the initial align-content: normal does not stretch tracks whose maximum sizing function is auto. This leaves unused block-axis space and places later rows too early.

Code OSS Welcome exposes the bug after named template areas are honored. Its 824px grid uses:

grid-template-rows: 25% minmax(min-content, auto) min-content;

Chromium computes 206px 600px 18px and places the footer at y=888 in the application viewport. The focused AppScene/WebScene package preserves the horizontal named-area geometry but places the footer around y=526 because the middle row remains at its intrinsic height.

Product-neutral reproduction

Use a 700x400 grid with rows 25% minmax(min-content, auto) min-content, a 200px-tall middle item, and an 18px footer.

Chromium 152 geometry:

{"grid":[0,0,700,400],"header":[0,0,700,40],"middle":[0,100,700,200],"footer":[0,382,700,18]}

WebScene native geometry:

{"grid":[0,0,700,400],"header":[0,0,700,40],"middle":[0,100,700,200],"footer":[0,300,700,18]}

The 82px of free space should enlarge the middle track's used size from 200px to 282px. The item itself remains 200px because it has an authored height; the following row moves to y=382.

Root cause

webscene_native_dom_layout.inc only performs row stretching when single_auto_row is true. The column path already distributes remaining inline space across stretchable automatic tracks, but the row path deliberately stops at the one-row compatibility case. It also needs to treat a minmax() track with an auto maximum as stretchable.

The portable arrangement and measurement paths should be audited for the same contract.

Proposed fix

1. Represent or preserve whether a minmax() maximum is auto; a resolved zero/unspecified length is not enough to distinguish it from other growth limits.
2. After intrinsic row sizing and fractional-track resolution, compute remaining definite block-axis space.
3. For align-content: normal/stretch, distribute positive remaining space equally across tracks with an auto maximum, including implicit auto tracks and minmax(<minimum>, auto).
4. Preserve intrinsic minimums, authored fixed/percentage tracks, gaps, fractional tracks, overflow behavior, and non-stretch alignment modes.
5. Apply the same rule in intrinsic measurement so measure and arrange cannot disagree.

Acceptance and regression gates

• Add a WPT-style contract for the exact three-row reduction above and compare exact Chromium/native rectangles.
• Cover more than one stretchable auto-max track, row gaps, minmax(0, auto), and minmax(min-content, auto).
• Cover align-content: start, center, space-between, and explicit stretch so the default rule is not applied to non-stretch modes.
• Cover an indefinite-height grid where no remaining-space stretch occurs.
• Add mutation tests for container height, row template, content intrinsic height, and align-content; assert synchronous geometry and no stale track cache.
• Add a bounded benchmark of at least 500 height/template mutations and gate both elapsed time and retained scratch-memory growth.
• Re-run the packaged Code OSS Welcome+Chat visual comparison. The footer should match the Chromium y=888 placement within the visual geometry tolerance while the horizontal named-area geometry remains unchanged.

Evidence

• focused package screenshot: artifacts/expanded-visual-audit-62a9647/welcome-chat-fixed/candidate.png in the Code OSS integration workspace
• Chromium reduction: /tmp/webscene-auto-track-chrome/results.json
• native reduction: /tmp/webscene-auto-track-native/results.json
• related named-area implementation: Implement CSS Grid named template areas for Code OSS Welcome layout #98 / PR Implement CSS Grid named template areas #105