FA-35888 / Tab interfaces / Member archive

Collapse empty pane: root placeholder · case 03

The tab workspace reports an incorrect root placeholder.

Member previewVariant 3 · 3 implementations · 7 checks per implementation

Case contract

A pane disappears only after its last tab closes; promote its sibling subtree, remove the separator, transfer active-pane ownership and retain sibling extent.

Why this case matters

Offline tab/panel workspace behavior; no browser or desktop framework is emulated.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
workspace regression 2	`[false, false, "p", 0, "p", 600]`	`[false, true, "p", 0, "p", 600]`	Failed

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