@jonaburg Next time, pls post related code so that we can help u properly... cuz we can't read your mind, boy! LOL
when tabbar on left or right:
#browser:is(.tabs-left, .tabs-right) #header {min-height: 0;}
when tabbar is not at top position:
#browser:not(.tabs-top) #header {min-height: 0;}
I don't really know what u really wanted, so I provide all possible solution that I know of... hope that can help u.
Oh, u might also want to add :not(.native):not(.is-settingspage):not(.popup) after #browser to exclude native window, settings dialog, & popup window from having no header/titlebar. 😉