DispatcherTimer.Tick can be executed when the timer is not running

[TomEdwardsEnscape]

Describe the bug

When DispatcherTimer elapses, it enqueues a callback on the UI thread. But other code can execute on the UI thread before that callback is executed. This other code can disable the timer, and reasonably expect that the callback will not be executed. But because the callback has already been added to the dispatcher's queue, it is unexpectedly executed.

To Reproduce

There is no way to precisely reproduce the issue, because it's a race condition.

Expected behavior

DispatcherTimer.Tick is never executed when DispatcherTimer.IsEnabled is false.

Avalonia version

master

OS

No response

Additional context

I worked around this issue in #15596. I didn't want to make a low-level change to timers for that PR.

[maxkatz6]

cc @kekekeks