Were the tabs movable like on BeOS? That’s something I cannot recall seeing in any theme.

I think so; it's been a while.

I seem to recall that you could hold down Alt key and drag the tab horizontally. So a tab could stick up anywhere along the top of the window frame. The SPARCStations had three-button mice, and this might have been mapped to middle or right mouse button.

The tabbed interface features the ability to gather windows into tab groups. So when you are moving a window by dragging its tab, if you drag the tab next to another window's tab, the dragged window will snap to that other window's frame, the dragged window will be the selected tab and its contents displayed in the frame. Then dragging any of the tabs drags the whole set of tabs that are snapped to that frame. Click on a tab to make it the active tab displayed in the frame.

It's effectively like tabs now, but sort of inside out: tabs are managed by the windows manager and not the app. Tab groups of multiple, different apps. The app does not have to be "tab-aware" to be in a tab group, although I suppose there are events that can signal the manager to make a window the selected tab. It's just raising a window, making it frontmost. Modal dialogs can get tangled up, but it's no worse than the mess commonly accepted today.