Well, MS Excel , Word etc. put a search widget and a user widget in the.title bar which occupy the whole thing. Moving Office windows is a pain.

Some occupied parts of the titlebar accept mouse-down-drag to move the window in MS Office apps, including:

document-name-dropdown, search-box, username, user-icon.

Elements that do not allow dragging the window, because they react to mouse-down, include:

application-icon, quick-access-toolbar, notifications-bell-icon, and the standard titlebar icons.

This requires experimentation, memory, and conscious thought moving a window did not require previously.

Visual Studio actually puts the menubar in the titlebar; I never have any place to click left unless the window is maximized (or, at least, full width).

Oh yeah, that's a little bit annoying. It looks like I can drag on the second toolbar on the very left but it would be much better to force some dragging space both left and right on the title bar. Otherwise I think menus there is okay, I don't need the entire title bar as a dragging area.

I'm ok with adding more functions to the titlebar, it's something I experimented with myself for a desktop app some 15 years ago. I found there was a heck of a lot of special behaviours tied to the titlebar and that overriding them meant a lot of work to fix edge cases like - being able to move windows. So I never followed through with it.

Maybe the addition of dedicated Move button next to the minimise, maxmimise and close button would be a reasonable compromise?

That would provide a consistent target to click on with your mouse. Obviously a button is a lot smaller than the entire titlebar for clicking on, so there would be some efficiency loss for people that regularly move windows (I am one of them). Getting move use out of the titlebar space would be worth the minor inconvenience of a more accurate click-drag to move operation.

If you need to use the move button, the move button needs to stay on screen. I may be the only one, but I regularly position windows partially off screen when I only need one side of the window and I don't have enough desktop space for the whole thing.

Browser tabs I actually find OK, I didn't even really consider them as "using" title bar space, since each tab is a title bar and contains no "active" areas that prevent the simple 'click and drag' action.

This does become more problematic when there are so many tabs they become tiny and the whole title bar of the browser is taken up by tabs so that there's no dead-space to drag the entire browser window to another screen. I try to keep minimal contexts (due to personal brain capacity issues), so I don't run up against this very often - I manage links that I want to keep and go back to using other means, or if I forget them then they weren't very important in the first place.