Aside from being draggable, the little gap gives an important visual clue, it spells out the hierarchy between window>tabs. I would guess that user testing backs this up, i.e. people are probably worse at completing tasks like "Move tab X from window A to window B" when the gap is removed.
If i remember correctly, that's so you drag & drop a full window rather than a single tab.
I'm looking at chrome with a title bar full of tabs. If I want to move the whole browser I have several places to grab the title bar. Additionally, from a sample size of one I find that selecting tabs happens with a far greater frequency than moving the window. Generally the widow is maximized on one display and I simply Alt-Tab to other windows.
Yeah, exactly. There's normally a bit of free space to the right of tabs. User can grab the title bar there. I'm switching between tabs like 1000 times more often than dragging the entire window anywhere. Perhaps I'm not a typical web browser user, but...
Or maybe it was window resizing. Either way, I believe that's the rationale behind it.