fix: memory leak in chat widget

[SimonSiefke]

Fixes a two memory in chat widget related to registering animation frames.

Details

When setVisible is called, an animation Frame Disposable gets registered using this._register(dom.scheduleAtNextAnimationFrame)), making the number of registered disposables grow over time

setVisible(visible: boolean): void {
  this._register(dom.scheduleAtNextAnimationFrame(dom.getWindow(this.listContainer), () => {
	  this._onDidShow.fire();
  }));
}

Similarly, for onDidChangeTreeContentHeight, a disposable also gets registered each time:

private onDidChangeTreeContentHeight(): void {
  this._register(dom.scheduleAtNextAnimationFrame(dom.getWindow(this.listContainer), () => {
	// Can't set scrollTop during this event listener, the list might overwrite the change

	this.scrollToEnd();
  }, 0));
}

Fix

The fix uses a MutableDisposable so that there can be at most one animation frame disposable registered.

Before

When sending a chat message and clearing all chat messages 37 times, the number of functions in ChatWidget.onDidChangeTreeContentHeight and AnimationFrameQueueItem seems to grow by 1 each time.

After

When sending a chat message and clearing all chat messages 37 times, the number of functions in ChatWidget.onDidChangeTreeContentHeight and AnimationFrameQueueItem stays constant:

[roblourens]

Ouch thanks!

[roblourens]

I think this can be merged with the one above but I can clean that up