Almost every Windows app now paints its menu bar or other chrome directly in what used to be the title bar space, and most of them, are custom and don't even count as "real" title bars to Windows. About the only thing to turn off these days is the space where Windows tries to paint the ordinary Min/Max/Close buttons, but even that can't be universal given how many apps today don't even use cutouts for that and instead draw their own. Looking at Fixefox, Slack, and Outlook on Windows 11 right now I see three different styles of Min/Max/Close buttons and I can't tell you which ones are really painted by Windows itself in a title bar cutout. Slacks look the most fake to my eyes because the icons are weirdly aliased and seemingly the wrong DPI, but that might not mean anything.