Where and how? On mine (downloaded a few days ago from stable) no option to show or use any tab bar.
Also, on Mac I can show the tab bar but can't get e.g. Ekos window to open as a tab (even when Kstars is full screen and checked Preferences "Independent Window" seems to make no difference to this).
No, not all pop ups. Only the applications that you would switch back and forth. A notification like covering the scope should be left as is. You typically just see those and close them. There would be no need to keep them active in a tab.
All of the independent windows (including the fits viewer) appear as tabs in full screen mode on the Mac version. Not sure why it's not working for some. I'm running Kstars 3.4.3 on Catalina 10.15.6.
Because I have two screens I prefer the concept of multiple windows.
There is only one annoyance: EKOS is minimized together with KStars' skymap window. In don't like this behaviour. EKOS should stay open if I try to minimize the skymap window.
I agree that too many windows is not a good thing. I think a good starting point would be to have fits viewer as part of capture tab, same way as with focusing and plate solving.
I'm running Kstars 3,5.1 with Catalina 10.15.7 and cannot replicate this window behaviour. "Independent Window" for Ekos/Indi viewers ON or OFF seemed to make no difference. In full screen mode Kstars filles the screen as expected. If Ekos is opened I get a floating window and once Indi is started it's in front of the Ekos window. Clicking the Ekos window doesn't cause it to move to front. I have to hide Indi (from KS tool bar) to see Ekos again,