Starting sometime after around 2023-06-27, Firefox Nightly will briefly flash some sort of popup window attached to each actual Firefox window when starting or in some other circumstances. It doesn't seem to happen every time a new window is created, but it may be each time a new window is created on a given URL (in the session). The only visible log message I can get when this happens is that it will always report:

JavaScript error: resource:///modules/PanelMultiView.sys.mjs, line 1191: TypeError: can't access property "GetAvailRect", screen is null

The flashing popup is sometimes delayed a bit from window creation.

My attempts at bisection of this with mozregression seem to point at an autoland that doesn't have any obvious relationship: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=c5ac7e957828eeda97a3af6fc706f41aa567e48e&tochange=eef8bff2d6117cc88dcca9ab4718be0f5186924b
It's possible that this is being masked by something.

My environment is Fedora Linux (37 and 38) on X11 using fvwm. I'm not using a compositing window manager, but starting xcompmgr doesn't make any difference to what happens.

The Bugbug bot thinks this bug should belong to the 'Core::Widget: Gtk' component, and is moving the bug to that component. Please correct in case you think the bot is wrong.

This appears to be related to bug #1838665. The popup that's flashing up seems to be the translation popup, and if I set browser.translations.automaticallyPopup to false, this stops happening. The web pages I'm visiting are in my own language. Since the code appears to offer this translation only once per host, it could explain why opening a new window doesn't always flash this translation.

Since the popup flashes up and then disappears immediately (generally too fast to read or react to), there may be a second bug here, if the popup is supposed to persist in this situation.

Setting Regressed by field after analyzing regression range found by mozregression in comment #0.

Set release status flags based on info from the regressing bug 1835629

:ochameau, since you are the author of the regressor, bug 1835629, could you take a look? Also, could you set the severity field?

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I also see this on Wayland fwiw. And I see it less than I used to but I still do...

I'm not using Linux as a daily driver. I have an Ubuntu installation I use for testing, but I'm unfamiliar with the X11 and Wayland particulars. I know that Erik uses Linux daily, but he hasn't seen this issue.

:emilio do you have any ideas on how we can reproduce this (even intermittently) on our machines?

I have also not actually seen this myself, both by chance and while trying to look for it.

:emilio I see that Greg didn't add an NI, so I'm adding an NI to see if you even at least remember an STR for where/how you saw this.

I see this sometimes after restart but not super-often... I don't have any precise steps to reproduce.

Bug 1846667 might be a duplicate of this, and bug 1846828 might help fix both.

Can those affected by this let me know if it's still happening. I found a reason errant popups were happening in Bug 1846828 and fixed it. Make sure to be on the latest Nightly. There is an uplift that is making its way to Beta as well.

This flash of a popup doesn't seem to happen for me any more on the current Nightly.