personally I hate the Mac 'finder' way of doing things... on a PC all that stuff is contained in each program's window which can be minimized or expanded as its totally separate thing...
If you have boot camp and a windows install, you might try a reaper portable install there to see the difference...
Otherwise... it's not a reaper problem... it's a Mac feature
Haha! There you go! Use an OS that doesn't have a usable menu bar in the first place and be none the wiser.
Isn't there a preference setting though to control the plugin focus options? I just haven't had the need to dig into that in my travels but pretty sure I've seen this mentioned.
I have VMware Fusion installed, but what I'm trying to monitor as I use Reaper is the battery level, which usually ends up around 15-20% by the end of the work day if I'm just running Chrome, WhatsApp and Reaper, in Dark Mode and with Night Shift maxed and on the lowest brightness setting (easy to still see 'cuz I'm on a dark stage). Running VMware during that time (which I have to do occasionally, if I need Samplitude) eats up the battery too fast.
Mac menu bar
Linear Mode
