User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:92.0) Gecko/20100101 Firefox/92.0

Steps to reproduce:

Open Thunderbird

Actual results:

The close, minimize adn restore icons on the top-right corner of the main window appear blurred.

Expected results:

the icons should look normal.

.

I can't test this as I have no Windows 7 Aero capable PC. Tweaking this rule https://searchfox.org/comm-central/source/mail/themes/windows/mail/primaryToolbar.css#281 to also on Win7 Basic or Win8 shows a correctly cut off button area.

Jorge, when you enable a theme that sets a background like https://addons.thunderbird.net/thunderbird/addon/thunderbird-suave/ is the button area correctly cut off?

installing that "suave" theme made things much worse, now the three buttons don't appear...

On the other hand, applying the dark theme fixes it. the buttons appear perfectly clear and cleanly cut.
the light theme works fine, as well.
it's the "system default" theme that causes the issue.

(In reply to Jorge Vidal Wulff from comment #3)

installing that "suave" theme made things much worse, now the three buttons don't appear...

This is what I thought. It was only for the test. In Troubleshoot mode (see Help menu) are the buttons still blurry?

troubleshooting mode looks fine.

Okay, then it's not a system theme issue. Do you use extensions or have something in userChrome.css that can interfere?

As a side note, Firefox also suffers from this (to a lesser extent though). applying the "alpenglow" theme makes things look A LOT BETTER.

I use three extensions: Xpunge, ImportExportTools NG and Lookout (fix version).
about userChrome.css, how can I tell if something is interferring???
But, like I said, the Dark and Light Themes look fine. Only the "system default" theme causes the issue.

btw, there's no "userChrome.css" in my TB profile...

Maybe bug 1729198?

Jorge, this still exist in version 102? (the core bug reports have not been marked fixed afaict)