Fix: Details layout rows render empty in grouped folders

[MemerGamer]

Resolved / Related Issues

• No issue filed. Found in a desktop test: Downloads (415 items) in Details showed only the folder icon and one or two clipped glyphs per row, with empty Date modified and Type cells and no group header.

Root cause

Grouping and grouped virtualization (#96, #100) are not involved. The Linux layout preferences database (layoutpreferences.json) serialized DetailsLayoutColumnItem.UserLength, which is a GridLength. System.Text.Json writes it as {IsAbsolute, IsAuto, IsStar} with no value. On load, UserLength comes after UserLengthPixels, so it overwrote every column width with default(GridLength) (Auto, 0).

The row template binds cell widths to LengthIncludingGridSplitterPixels, which came out as 0 or 12 px, so the cells collapsed. The header uses GridLength directly and still looked right. The zero widths were then saved back, so the folder stayed broken. Any folder whose preferences were loaded from disk hit this. The reported Downloads entry had zero widths and no grouping (it was saved before #96 restored the grouping default), which is why no group header appeared.

Fix

• UserLength is now [JsonIgnore] (it was already [RegistryIgnore]). The width is persisted only through UserLengthPixels.
• When loading or importing the Linux database, a column width of 0 or less goes back to the default width from settings (24 px for the icon column). A width of 0 is never valid because hidden columns are tracked separately. This repairs files that earlier builds already corrupted.

Steps used to test these changes

All runs used headless-run.sh with a sandboxed HOME and FILES_SANDBOX_DRIVES=scripts/linux/showcase-drives.txt. Downloads was seeded with 380 long-named folders and 35 files, with modification dates spread over about 400 days, and the app started directly in Downloads.

1. Before the fix, a valid persisted Details entry for Downloads reproduced the report exactly: icon plus clipped glyph, empty cells. After that run the file held UserLengthPixels: 0.
2. After the fix, an already corrupted entry (zero widths) with grouping by date modified renders name, date, type and size correctly. Switching to Grid, then List, then Cards, then back to Details all render correctly. The saved file has default widths and no UserLength.
3. After the fix, a valid entry without grouping renders correctly. A second session that loads the file written by the fixed build also renders correctly.
4. Earlier runs without persisted preferences (grouped by date, all-today dates, resize under kwin_x11) were already correct and still are.
5. tests/Files.Platform.Tests: 1055 total, 1048 passed, 7 skipped, 0 failed. Desktop app build succeeded.

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