[foxnne]

Stacked on #465 (it reuses that PR's ColumnCardFrame test frame); review after it, or retarget to main once it merges. This is the follow-up #465 named.

What changes

Take Main | (Main/r1 over Main/r1/b1). Emptying Main/r1 (carrying its last view to the trash or elsewhere), or picking Remove on it, now closes just that half. b1's views move into Main/r1, and the column keeps its name, its width and its link in layout.zon. That's the same rule as emptying the half that was split anywhere else (SPLITS.md, "Removing a split"). If b1 is split again too, there's no single place to merge into, and Main/r1 stays, empty.

Before: Main/r1 is two places under one name: the half Main's split minted, and the half its own split kept. closeEmptied saw "minted" and slid shut the branch box registered under that name, which is the whole column. Then Forest.collapse("Main/r1") matched the inner leaf, so b1 was promoted into Main's split. b1 took the column at an extent that was its height (195 pt in the test). The only link left in layout.zon still named the gone Main/r1, and b1 had no link, so it was lost on reload.

The model change:

• SplitTree.Forest.collapse takes the outermost node that carries the name: for a place split again, that's the branch its split made, never the leaf that kept the name. New isSplit and placeLeaves read the same rule.
• ViewDrag.closeEmptied treats a minted half that was split again as an origin: it merges in its other half, or stays. The Picker's Remove goes through it for such a place.
• Region.persistExtent counts a column as kept while any place in it holds a view. When the column closes, it forgets every place in it, not only the one whose name the column carries. Before, closing a column with an empty Main/r1 and a full b1 would have left b1's views assigned to a place that no longer existed. It walks the forest only once the place has shut, not every frame.
• The picker offers no Remove on a half that was split again when its other half is split too: there's nothing to merge, and Remove would only have cleared it.
• A use-after-free while drawing. When a place was forgotten in the middle of drawing its branch, the tree went on drawing children that collapse had just freed. The old code had the same hazard on a rarer path. persistExtent now returns whether it dropped the place, and initTree and the .branch arm draw nothing more of it that frame (a refresh is already queued). The new column-close test caught this: the freed b1 leaf re-stored its extent after the column had cleared it.

A leaf that was never split again behaves exactly as before. Every changed path checks isSplit first, and placeLeaves of a plain leaf is that leaf alone.

SDK impact

• None (app/ only; test-sdk-version passes)

Verified

• macOS:
  • integration tests (new) on fizzy's forest with a carded Main:
    • split: emptying the half that was split again closes only that half. It failed before the fix: Main/r1 was gone and b1 sat in Main's split.
    • split: emptying the half that was split again stays when its other half is split too.
    • split: a column shut by its sash goes only once every place in it is empty: shut with a view in b1, it stays; emptied, the whole column goes and leaves no assignment or extent behind.
  • unit tests in SplitTree.zig: collapsing a place split again drops its whole branch, and collapsing the minted half of a nested split leaves its origin, with one link.
  • sandbox fizzy (--profile, through the agent plugin, macOS 2×):
    • Main → Split → Vertical gives 870 | 870 px.
    • Main/r1 → Split → Horizontal gives 478 / 478 px, with the column's extent unchanged at 435 pt.
    • Output into b1, then Main/r1 → Remove gives Main 870 | Main/r1 870 px. Main/r1 holds Output, and layout.zon has one link (Main/r1 from Main, right) and no b1.
    • Remove on b1 itself still closes just b1, as before.
  • scripts/check-fmt.sh, zig build, zig build test (91/91 steps), zig build test-integration (53/53 steps), zig build check-tests-reached, zig build check-web, zig build test-sdk-version.
• Windows:
• Linux:
• Web: builds

Follow-ups

• The deeper fix is a name of its own for a branch. A branch and the half that kept its name share one name, and this PR special-cases that (isSplit) where it bites: closeEmptied, the picker's Remove and persistExtent. Anything else that resolves a place by name (regionNamed, which finds the branch box first; placeSize) still sees two places. Naming branches separately would need a layout.zon migration for saved links, so it's its own change.

• Picker buttons named for drivers: layout: the picker's buttons are named for tapes and agents #468.

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