[regularfry]

If they move the mouse while holding the button, how do you tell the difference between trying to move the window, and trying to move just that one tab?

[kijin]

When you're trying to reorder tabs, you are much more likely to click on the tab itself, not the pixel above it. Try it yourself, it happens naturally. Drag-and-drop usually involves more precise mouse control than simple clicking.

So the software can safely assume that if you drag the top pixel, you're trying to move the window rather than the tab.