When an extension creates a window type=panel, if the browser is closed while the window is open, upon session restore the window opens in a broken state, with no titlebar.

In some cases, if the addon has a sidebar ui that is open when the browser is shut down, the restored panel will appear with the sidebar. However, while I and other users have observed the sidebar appearance behavior, I have not been able to consistently repeat this. However the no-titlebar effect is consistently repeatable.

To use the attached demo addon:

1. Create new profile
2. Install the extension
3. Click on the green browserAction toolbar button
4. When popup opens, click the "open panel" button
5. Observe that panel opens
6. Close out and restart Firefox
7. Observe the panel appears without a titlebar

Either the panel should not open in a broken state, or SessionStore should not reopen panels.

Bugbug thinks this bug should belong to this component, but please revert this change in case of error.

Related: Bug 1660564

The severity field is not set for this bug.
:mikedeboer, could you have a look please?

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I can reproduce this issue on Firefox Nightly on macOS.

I can reproduce this on Windows 10 and on Linux Mint 20.2 MATE

(In reply to Asif Youssuff from comment #4)

I can reproduce this issue on Firefox Nightly on macOS.

Could you please set the bug's priority and severity?

This will be fixed in bug 1564738.

The no-titlebar effect was fixed by bug 1564738, but the intermittent sidebar appearance behavior is not.