I notice this happens only when the window is maximized, since otherwise you have gap above the tabs.
I don't think it's a bug. I think is is done so that you can move the window by dragging that one pixel bar.
In both Firefox and Chrome if you want to move the window when you have the window maximized, dragging on the top of the tab detaches the tab from the current window. The only way to move the window is to move the cursor to a space where you don't have a tab or by clicking on the restore button (the middle button on top right).
When you have lots of tabs open then there is no blank space on the right of the tabs to hold the window and move it.
Usually, I've noticed Opera users to have lots and lots of tabs open, which doesn't leave any blank space on the right of the bar. At that time this one pixel is a life saver to move the window without needing to go all the way to the top right to restore the window.
"The only way to move the window is to move the cursor to a space where you don't have a tab"
This may be true in whatever window manager you're using, but it's not true for all of them, including the one I use (dwm). Better window managers let you move and resize windows without having to click on special places that may or may not be provided by the programs that draw the windows.
I don't think it's a bug. I think is is done so that you can move the window by dragging that one pixel bar.
In both Firefox and Chrome if you want to move the window when you have the window maximized, dragging on the top of the tab detaches the tab from the current window. The only way to move the window is to move the cursor to a space where you don't have a tab or by clicking on the restore button (the middle button on top right).
When you have lots of tabs open then there is no blank space on the right of the tabs to hold the window and move it.
Usually, I've noticed Opera users to have lots and lots of tabs open, which doesn't leave any blank space on the right of the bar. At that time this one pixel is a life saver to move the window without needing to go all the way to the top right to restore the window.