[datlechin]

Summary

• A Structure tab section grid mounts at height 0, then grows once it has measured its rows. An accessibility read in between (VoiceOver, or the UI test polling right after showStructure) prepares every row while none is on screen, so no row gets its data-grid-cell stand-in, and AppKit never asks for a prepared row again. Growing is not a scroll, so nothing remounted them.
• That is the red StructureSaveConfirmationUITests on main: all four failure trees (runs 40967120404, 40966363060) show the only data-grid with its 3 rows and no stand-in, so the test was not matching a different grid and the columns did load.
• The grid now reconciles stand-ins from the rows themselves instead of a cached range, after its own layout and after each read of the table, besides scrolls.
• .claude/rules/data-grid.md, Accessibility: when on-screen rows remount.
• No test changed: the UI test pins correct behavior. No changelog fragment: the fitted section grids came with feat(structure): rebuild the Structure tab as one Definition page with an inspector #3872, after v0.84.0-beta.1.

Test plan

• DataGridAccessibilityMountTests, 25 repetitions each: on main, "read before it grows" and "moved onto the page" fail 25 of 25 (no row ever mounts); here all three pass 25 of 25.
• DataGrid and Structure accessibility neighbors (DataGridControllerTests, StructureListGridAccessibilityTests, DataGridFittingHeightTests, DataGridViewportTests, DataGridEditingTests, DataGridMountTests): 91 of 91.
• StructureSaveConfirmationUITests runs in CI's UI plan; Automation Mode is off on the dev Mac.