[stable5.7] fix(drag-and-drop): track all droppable-mailbox directive instances independently

[backportbot]

How to test:

• Click on a message and hold
• Drag the message to a folder to move it

Main: It's not possible to drop on the latest mailbox/folder on the list
Here: It works

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Backport of #12650

Warning, This backport's changes differ from the original and might be incomplete ⚠️

Todo

• Review and resolve any conflicts
• Review and verify the backported changes
• Amend HEAD commit to remove the line stating to skip CI

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[kesselb]

Backport does not apply clean because DroppableMailboxDirective looks different on stable5.7 due to #12596.

Partial backport should be fine, or would you prefer to drop the vue3 definitions @ChristophWurst?

[Copilot]

Pull request overview

Backport to stable5.7 that fixes sidebar drag-and-drop by tracking each v-droppable-mailbox directive instance independently and cleaning up listeners on unmount.

Changes:

• Replace the single shared directive instance with per-element instances stored in a WeakMap
• Ensure listeners are removed on unbind/unmount and that removeListeners uses the same bound function references
• Add unit tests covering multiple instances, updates, and cleanup behavior

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/tests/unit/directives/droppable-mailbox.spec.js	Adds unit tests validating independent instances, updates, and listener cleanup
src/directives/drag-and-drop/droppable-mailbox/index.js	Refactors directive to track instances per element and adds unbind/unmounted cleanup
src/directives/drag-and-drop/droppable-mailbox/droppable-mailbox.js	Stores bound handler references so event listeners can be removed correctly

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[Copilot]

DroppableMailbox is constructed here with only (el, binding.value), but the class constructor signature is constructor(el, componentInstance, options). This means options becomes undefined and options.mainStore will throw at runtime. Pass binding.value as the third argument (and optionally provide the Vue component instance as the second arg for Vue 2/3), or update the DroppableMailbox constructor to match the new call site.

Suggested change

	const instance = new DroppableMailbox(el, binding.value)
	const instance = new DroppableMailbox(el, undefined, binding.value)