Hidden iframes sometimes don't get recomposited properly

[jdm]

iframe1.html

<iframe src="iframe2.html"></iframe>
<script>
var iframe = document.getElementsByTagName('iframe')[0];
iframe.onload = function() {
  iframe.style.display = "none";
}
</script>

iframe2.html
Iframe content

The iframe border disappears, but the content sticks around consistently for me.

[jdm]

Blocks #6677

[jdm]

@glennw @mrobinson As far as I've been able to trace this, we correctly remove the compositor layer when we set a construction result of None. The constellation code correctly sees the pipeline in the current frame in the resulting FrameRect message, and sends the compositor a SetLayerRect message. The compositor finds the layer and updates the bounds, but send_buffer_requests_for_all_layers ends up with an empty layers_and_requests vector and returns without requesting new data from the pipelines.

[mrobinson]

I think this is a regression from servo/rust-layers#182. Layers can either mask their children or not and iframe containers always mask their children. Now, it seems that the property is only respected for layers that create a new RenderContext3D.

[glennw]

In theory, iframe root layers should always create a new RenderContext3D, which should handle that situation correctly. But perhaps there's a bug and iframe root layers don't result in a RenderContext3D being created?

[mrobinson]

Okay. After some investigation I have a fix lined up for rust-layers (servo/rust-layers#201). Once that lands I'll do a PR for servo with a test.

[mrobinson]

An update on this one. I think the rust-layers PR fixes the bug, but I'm having a bit of trouble writing a non-flaky reference test for this issue. It is very sensitive to timing. :/