Dismiss file-drop hints when the drag interaction ends

[austinywang]

The “Hold Shift to open as split” hint could outlive a file drag when its destination window lost focus or AppKit omitted the usual exit/conclude callbacks. The hint now belongs to the native drag sequence and dismisses on drag end, drop preparation, mouse release, keyboard input, window resignation/closure/detachment, another window becoming key, or app deactivation. Later updates from that same sequence cannot revive it.

Fixes #12925.

The presenter owns only feedback. Focus loss during a browser drop leaves WebKit's prepare/perform/conclude delivery intact. Mouse-up and Escape pass through unchanged so AppKit can finish/cancel the source drag and remove its filename image.

Trade-offs: the hint dismisses immediately rather than fading, and a single eight-second display deadline also hides it during a long, otherwise valid drag. The existing deadline timer is used for this intended UI lifetime; there are no sleeps or polling in app code. The generic badge, file-drop preferences, and keyboard shortcut settings are unchanged. FileDropPaneTarget moved into its own file to keep the existing large file within its budget; neither budget TSV changed.

Validation:

• Regression commit c58c70840a1db0bbddfc0c267b6b541a77a67fb8: gh workflow run test-e2e.yml --repo manaflow-ai/cmux -f ref=12925-split-hint-stuck -f test_filter=cmuxTests/FileDropOverlayViewLifecycleTests -f record_video=false -f runner=blacksmith-6vcpu-macos-15. Run 36094176207 executed four tests and failed. The three window/application lifecycle assertions demonstrate the missing teardown; the fourth asserted synchronously before the old exit animation finished.
• python3 scripts/verify-local.py --swift-changed origin/main: Swift syntax, project normalization, test wiring and feature-flag policy passed. ./scripts/lint-pbxproj-test-wiring.sh and python3 scripts/swift_file_length_budget.py passed.
• ./scripts/localize-changes --base origin/main: no changed UI strings; all nine required catalog locales passed.
• Final focused test execution and exact-head controller build are pending.

Live verification is incomplete. A tagged baseline build succeeded (job a65f7659609271497a20f2ae, SHA 1eee9111527de24fd06965cae8e482354ebe7017), but the cross-application drag attempt left a visible drop.txt image on the shared desktop. That observation does not establish the reporter's exact browser trigger or prove the source-image problem is repaired. Escape was sent to Finder; no stable-app restart or destructive cleanup was performed. The final native drag-image cancellation still needs verification. Cloud Mac preflight is blocked by its retired macfleet dependency; local full-display recording lacks Screen Recording permission. Baseline HQ publication also failed local certificate verification; controller artifact download succeeded with SHA-256 verification, and feedback 403dc8664c345d15eec622c2 records the publication failure.

— JuniperEmber (registration pending)
run: run_issue_12925_split_hint_20260925
session: cmux144-12925-20260925

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Summary by cubic

Fixes the "Hold Shift to open as split" hint sticking around after a file drag ends, when the destination window lost focus or AppKit skipped its usual exit/conclude callbacks. The hint is now tied to the native drag sequence and dismisses on drag end, drop preparation, mouse release, keyboard input, window resignation/closure/detachment, another window becoming key, or app deactivation. Later updates from the same sequence cannot revive it.

Fixes #12925.

Behavior

• Hint dismissal doesn't cancel WebKit's drop delivery; mouse-up and Escape pass through unchanged so AppKit can finish/cancel the source drag and remove its drag image.
• The hint dismisses immediately instead of fading and is hidden by an eight-second display deadline even if the drag continues.
• FileDropPaneTarget moved into its own file to keep the existing large file within its budget.

^{Written for commit 55e4147. Summary will update on new commits.}

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📥 Commits

Reviewing files that changed from the base of the PR and between 85a3655 and 55e4147.

📒 Files selected for processing (7)

• Sources/FileDropHintPresentation.swift
• Sources/FileDropOverlayView.swift
• Sources/FileDropOverlayViewHitTesting.swift
• Sources/FileDropPaneTarget.swift
• cmux.xcodeproj/project.pbxproj
• cmuxTests/FileDropOverlayDragCompletionTests.swift
• cmuxTests/FileDropOverlayViewLifecycleTests.swift

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