I think this probably only shows up with fission oop iframes, tabs probably avoid this path.
The problem occurs when we reconstruct the containing iframe for a style change, we briefly hide the child document, clearing the display list on the parent via ClearCachedResources. Then show it again, we attempt an empty transaction and this succeeds because there is nothing to stop it. (The non-wr case fails because the layer contents are missing and that causes the empty transaction to fail.)
So keep track if we have sent a display list to the parent to allow/disallow an empty transaction.
This fixes a couple webrender+fission reftest failures but it's also a general rendering bug in webrender+fission reproducible in a regular browser.
Differential Revision: https://phabricator.services.mozilla.com/D61577
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extra : moz-landing-system : lando
68c2b57363