User Agent: Mozilla/5.0 (iPhone; CPU iPhone OS 14_7_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.1.2 Mobile/15E148 Safari/604.1

Steps to reproduce:

Create new e-mail.

Actual results:

Looks ugly with 4k resolution and 150% scaling.
Some fields are overlapping.

Expected results:

Should look normal without overlapping fields.

Can you provide a screenshot?

Sure, there are dark grey borders - no overlapping - sorry for misspelling.

Are you sure you don't have any userChrome.css modification, or add-ons/themes messing things up? Try starting with -safe-mode

I don’t have any modifications or addons/themes - but in safe-mode it looks normal.
It looks normal in Thunderbird 78 too.

The Thunderbird dark-theme looks also normal.
It’s just the Thunderbird System-Theme.

If it looks normal in safe-mode, there's some theme/add-on causing it.

Weird, in safe-mode it uses the system-theme. Do you use the light theme normally?

I organized a 4k monitor and tried your STR. For me it is still looking correct with all themes. Do you have something in the userChrome.css?

It occurs only with Thunderbird Systen-Theme which is used by default.
Dark and Light look ok.

I dont‘t have a userChrome.css.
It‘s a fresh installation.

Additional info:

Problem only occurs at 150% display scaling.
The dark grey border doesn‘t appear at 125% or 175% scaling.

I tested it with 150% and it looked correct. Windows sets this automatically whit new 4k monitors.

When it is in safe mode okay it seems it's a config that interferes. Could you try to disable HW acceleration (Preferences/General on bottom)?

It looks ok with all themes if I disable hardware accelaration and restart Thunderbird afterwards.

Graphics driver is this one:

https://www.dell.com/support/home/en-us/drivers/driversdetails?driverid=h2trr&oscode=wt64a&productcode=xps-15-9510-laptop

Thanks. Not much we can do about that.