I'm having this same issue and have been having it since I bought QB2019 for my Mac. I did not upgrade.

As I read through this conversation, I got confused. It starts out focusing on the Mac version, but at some point, it looks like it switched into the Windows version. For example, there is no little box in the upper left corner w/ the minimize/maximize/close buttons being on the right. This is definitely not a Mac screen shot.

Yes, I'm very frustrated by this behavior of the windows. I have told QB to open all windows as tabs in 1 window and I generally have 2 windows when I open QB, and have to tell it to merge the windows again. I've also noticed that some things that I'm displaying in the window do change the size of the window. For example, the Vendor and Customer Centers make the window grow beyond the size of the screen. You can change its wide, but you can't change its height. If I click on a tab that is displaying a bank account, I can resize the window any way that I want to. But then if I click on the Vendor or Customer Center, BOOM!, I'm back to a window that's too big for the screen. This is extremely annoying! This doesn't make any sense, and Intuit should be able to fix it.