Keep the main window floor on the animating setFrame path

[teamleaderleo]

Summary

Split down, split up, then resize the window to 800x200: one of the three stacked panes ended at 0x0 (#15347).

The main window has a 400 pt minimum height, and CmuxMainWindow enforces it by overriding setFrame(_:display:). setFrame(_:display:animate:) is a separate entry point that does not route through that override, so display placement, UI-test placement and the debug resize verb could size the window below the floor. The workspace content cannot shrink past its own 400 pt minimum, so it overflowed the window: the pane area stayed 372 pt tall inside a 200 pt window, and the bottom pane was laid out entirely below the window edge, where the terminal portal collapses it to 0x0.

The fix is to apply the same size policy (cap to the display union, raise to the floor, leave full screen alone) on the animating path too. The window now stops at 800x400, the pane area fits, and each of the three panes keeps a real height.

Pane-level clamping was the other option. bonsplit already keeps both children of every split nonzero within its container; the zero came from the container being taller than the window, so no pane minimum could fix it. Keeping the window at or above its content minimum removes the whole class of overflow, not just this split shape.

Testing

• Added MainWindowSelfSizingTests.testAnimatingSetFrameRaisesUndersizedFrameToMinimumContentSize: an 800x200 setFrame(_:display:animate:) must keep the width, raise the height to the floor and keep the top edge.
• Added the fuzzer's repro as dogfood/fuzz/regressions/issue-15347-stacked-panes-short-window.json, so scripts/fuzz regressions --app ... replays it.
• Replayed the repro on a build mini: main reproduces pane-view-degenerate at step 3; this branch passes (frames below).
• CI ran cmuxTests/MainWindowSelfSizingTests (with AppDelegateFullScreenFrameRestoreTests and MainWindowZoomPlacementTests) in the app-host changed-suites job on this head: passed. The one red guard test, test_ci_owned_spm_scratch, comes from ci: bound the SwiftPM scratch holder and cache scratch sizes #15366 on main and is unrelated to this change.
• Five stacked splits leave panes shorter than their tab bar even at full size, before any resize. That is a separate split-sizing problem, filed as [fuzz] Repeated splits shrink a pane below its tab bar, leaving a 0 pt terminal #15371.

Changelog

Fixed: Resizing the main window from a script or a display change can no longer shrink it below its minimum height and leave a split pane with no size

Demo Video

The fuzzer repro (split down, split up, resize to 800x200) replayed on a build Mac against a DEV build of main and one of this branch, with a frame after each step.

Main, after the resize: the window is 200 pt tall, the pane area still lays out at its 400 pt minimum and overflows, and the third pane is off the bottom edge at 0x0 (the replay reports pane-view-degenerate).

This branch, after the same resize: the window stops at 800x400 and all three panes keep a real height (the replay passes).

Before the resize, for reference (this branch):

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 1a7467a and 781dd46.

📒 Files selected for processing (3)

• Sources/App/CmuxMainWindow.swift
• cmuxTests/MainWindowSelfSizingTests.swift
• dogfood/fuzz/regressions/issue-15347-stacked-panes-short-window.json

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Dogfood build of 781dd46555119425e41cdd561f303d615ec29d7a

cmux DEV pr-15368-781dd465.app

The link opens this exact commit in the cmux dev menu bar app. The build starts on each push and the page waits until it is ready; a newer push replaces it. It signs in against production, so Cloud or backend changes still need a tagged build with a development backend.

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CI passes on 781dd46555 (run 36426636081 attempt 3).

Written by scripts/ci/classify_failures.py (ci-failure-attribution.yml); signatures are its SIGNATURES table. A machine verdict is the runner's fault, not this PR's.

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Merge receipt for 781dd46555: every check was green at merge (16 verified; 16 skipped by policy). Full suite runs on main after merge.