Background of toolbar doesn't support some themes when set to transparent

[RealRaven2000]

When starting Thunderbird 91 with themes like "Earlybird" the toolbar background doesn't look correct. When using the "transparent" option the desktop shines though.

Example:

once we click somewhere in the Thunderbird UI, then the picture from the theme is painted:

Expected behavior: the picture should show immediately on starting Thunderbird.

[RealRaven2000]

Just confirming, this only happens with Thunderbird 91. Also when another Theme is selected during the session the same issue happens again. We need to find out whether there is an event we can hook up to for rectifying this.

[jobisoft]

related?
https://thunderbird.topicbox.com/groups/beta/T60a437d3b4488869/thunderbird-93-and-earlier-compose-window-rendering-issue

[RealRaven2000]

related?
https://thunderbird.topicbox.com/groups/beta/T60a437d3b4488869/thunderbird-93-and-earlier-compose-window-rendering-issue

Interesting, may well be related! I think it's some css rules for background colors or background images that don't get triggered in the rendering engine. The problem is that as soon as I use Inspector and click on my toolbar the problem fixes itself...

[RealRaven2000]

Also in Thunderbird 91.1.0 in Windows 7 the minimize / maximize / close buttons are painted over by lightweight themes with images: