ImageAnimator.StopAnimate (on Unix) could throw PNSE

[jeffhandley]

With dotnet/designs#139 and #39269, we are marking Thread.Abort() as Obsolete because it unconditionally throws PlatformNotSupportedException. The tests for the PR turned up that ImageAnimator.Unix.cs uses Thread.Abort() inside StopAnimate.

        public static void StopAnimate(Image image, EventHandler onFrameChangedHandler)
        {
            if (image == null)
                return;

            if (ht.ContainsKey(image))
            {
                AnimateEventArgs evtArgs = (AnimateEventArgs)ht[image]!;
                evtArgs.RunThread!.Abort();
                ht.Remove(image);
            }
        }

I am going to put a #pragma warning disable around the Thread.Abort() call, but this code would be unexpectedly throwing PNSE right now, which means it's either dead code or there's a lurking bug that no one has reported.

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[jeffhandley]

Area owners (@maryamariyan @michaelgsharp @safern @tarekgh) -- Do you think it would be appropriate to mark ImageAnimator.StopAnimate as [UnsupportedOSPlatform("unix")] since it's going to throw a PNSE if the image is animating? Or do you think we'd want to find a new way to implement animation stops such that it's not based on aborting threads?

[tarekgh]

I prefer if there is a way to avoid using Windows specific Thread.Abort and find equivalent on Linux. I am not even sure if Thread.Abort is the best option for Windows either.

CC @jkotas

[jkotas]

Add a bool flag that gets set by StopAnimate and the animation loop checks every iteration. It will match what the implementation does on Windows.