CanvasRenderingContext2D.drawWindow() is synchronous and won't work with fission, so WindowsPreviewPerTab.jsm[1] must be updated to use the asynchronous drawSnapshot[2]

1. https://searchfox.org/mozilla-central/rev/0bcf81557b89e7757c44e25bb4bc7f4cb8619dc9/browser/modules/WindowsPreviewPerTab.jsm#308
2. https://searchfox.org/mozilla-central/rev/9b282b34b5/dom/chrome-webidl/WindowGlobalActors.webidl#81-98

Tracking for Fission Beta milestone (M7).

I am vaguely under the impression that WindowsPreviewPerTab.jsm is only used in the parent process, where IIRC drawWindow may still work correctly (though please correct me if I'm wrong on that :mattwoodrow). We may not need to make changes here?

If we're calling from the parent process, and only trying to capture content from the parent process (which it appears that we are, from the code linked in comment 0), then this will continue to work fine.

Does comment 3 mean we can close this? Can we look at all WindowsPreviewPerTab.jsm usage to see if it's all from the parent?

Gijs, are you the right person to confirm that this will only run in the parent process? Thanks!

(In reply to Matt Woodrow (:mattwoodrow) from comment #5)

Gijs, are you the right person to confirm that this will only run in the parent process? Thanks!

Yes, it will only run in the parent - https://searchfox.org/mozilla-central/search?q=WindowsPreviewPerTab shows it's only loaded from browser.js, which only runs in the parent process.