LGTM with a couple of tiny suggestions :)

Thanks for the fix and the well-structured test!

Thank you for leaving a comment. I am concerned about the changes to markNeedsPaint, and we might need someone more familiar with RenderObject to review it.

@dkwingsmt Is this something you would be up for reviewing?

Thanks for the contribution. The change seems reasonable to me. But since this involves a change to RenderObject, I think we should add a test for RenderObject so that this fix is verified even if AnimatedOpacity is gone.

Also cc @jonahwilliams who worked on related code 3 years ago and should be more knowledgeable than me.

Side note

It took me a while to understand the relationship between markNeedsCompositedLayerUpdate and markNeedsPaint:

So basically, other than the first few lines, the rest of the two functions are identical. I suggest we refactor it somehow to indicate it more clearly. (It doesn't have to be done in this PR.)

But since this involves a change to RenderObject, I think we should add a test for RenderObject so that this fix is verified even if AnimatedOpacity is gone.

@dkwingsmt Tests for RenderObject have been added, feel free to review.

auto label is removed for flutter/flutter/157663, due to - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.

BTW – I've learned that Google testing can have a hard time understanding merge commits, so it might be better to do a rebase here

https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#using-git

@dkwingsmt Did the Google testing fail because of this assertion?

Yeah it was. But I don't think we can remove this assertion, otherwise it means this change will change the logic of existing apps. Let me see if I can find why it was triggered.

I wonder if it's because markNeedsPaint is called after _nodesNeedingPaint is used before _needsCompositedLayerUpdate is reset.

Edit: I found that a handful of tests are broken and seem to be caused by the exactly same widget. I'll look more into it.

I took a look and propose a different fix:

// object.dart, L2964
-       owner?._nodesNeedingPaint.remove(this);
+       owner?._nodesNeedingPaint.removeWhere((RenderObject t) => t == this);

This fix works for both your unit test and the internal tests.

Analysis: The reason why adding the object to owner!._nodesNeedingPaint twice causes the problem is because while flushCompositingBits is supposed to clean up all objects that have become non-compositing, the job fails because only the first encounter is removed. Therefore flushPaint unexpectedly encounters non-compositing objects.

And the reason why I think the current approach might not be the best is because the system is so complicated that we can't assume _needsCompositedLayerUpdate == true is equivalent to owner!._nodesNeedingPaint.contains(this).

And it seems _nodesNeedingPaint is designed to allow duplicates, since flushPaint checks whether a node still _needsPaint on each iteration afresh.

If you use this approach, maybe we should add a comment to _nodesNeedingPaint to explain that there might be duplicate nodes.

I have committed the solution you proposed, If _nodesNeedingPaint allows duplicates, then we can handle it this way.

@jonahwilliams Would you like to take another look, since we've changed the approach?

I'll wait until after the US thanksgiving holiday to settle this PR. Thanks for your patience.

auto label is removed for flutter/flutter/157663, due to - The status or check suite Windows build_tests_1_9 has failed. Please fix the issues identified (or deflake) before re-applying this label.