fix: clear terminal drag-and-drop observer on detach (fixes #311164)

[Suryaansh Prithvijit Singh (Ultron09)]

Summary

Fixes #311164.
Supersedes #318756 with requested unit tests and race condition guards.

TerminalInstance._dndObserver (a MutableDisposable<IDisposable> holding the per-container DragAndDropObserver) was set by _initDragAndDrop but never cleared by detachFromElement. When a TerminalEditor pane reuses its container across tab switches (setInput calls detachFromElement on the old instance and attachToElement on the new one), newly-attached terminals stacked additional DragAndDropObservers on top of leaked ones.

When shift-dropping a file onto the active editor-area terminal, each previous instance's observer fired on the shared container and inserted the path.

Changes

1. Dispose this._dndObserver.clear() in detachFromElement() so drops on the old container no longer route to the detached terminal instance.
2. Guard the deferred setTimeout in attachToElement with if (this._store.isDisposed || this._container !== container) return; so a synchronous detach in the same tick does not re-attach a stale observer.
3. Added unit tests in terminalInstance.test.ts verifying that detachFromElement properly clears the observer.

Co-authored-by: Vlad Gerasimov vlad@vlad.studio

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

Anthony Kim (@anthonykim1)

Matched files:

• src/vs/workbench/contrib/terminal/browser/terminalInstance.ts
• src/vs/workbench/contrib/terminal/test/browser/terminalInstance.test.ts

[Copilot]

Pull request overview

Fixes leaked terminal drag-and-drop observers when editor containers are reused.

Changes:

• Clears the observer when detaching.
• Guards deferred initialization against stale containers.
• Adds detach cleanup coverage.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
terminalInstance.ts	Cleans up and guards drag-and-drop observers.
terminalInstance.test.ts	Tests observer cleanup during detach.

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[Suryaansh Prithvijit Singh (Ultron09)]

Added a unit test in terminalInstance.test.ts verifying that attaching and immediately detaching in the same tick prevents deferred _initDragAndDrop execution once the timer resolves.

[Dmitriy Vasyura (dmitrivMS)]

I prepared stronger regression coverage that creates real TerminalInstance objects and dispatches an actual file drop against a reused container. It verifies both detaching after the observer is initialized and detaching before deferred initialization runs.

I couldn't push to the contributor fork because maintainer edits are unavailable. Please cherry-pick b1b63b784ac into this branch.

[Dmitriy Vasyura (dmitrivMS)]

Replacing with #335945 which has better unit-tests.

[Suryaansh Prithvijit Singh (Ultron09)]

Thanks Dmitriy Vasyura (@dmitrivMS) for the review, approval, and for strengthening the regression coverage in #335945! Left a quick note on #335945 to please include Co-authored-by: Ultron09 <connect.singha@gmail.com> when merging. Much appreciated!