Scrolling selection with mouse doesn't work if mouse is moved beyond window borders

[nh2]

Ubuntu 16.04, Kitty 0.12.0.

When I click-and-drag-up to select text, scrolling up with the selection only works if I keep the mouse in the top couple pixels of the kitty window.

If I move it further, e.g. into the window decoration or beyond, scrolling stops.

This makes it very hard to scroll up, because I have to carefully keep the mouse in the relevant region.

Other terminal emulators don't have this issue, they continue scrolling up also when the mouse is in the decoration or even above it.

Kitty 0.1.0 also didn't have this problem, scrolling would usually continue even beyond the decoration, but that version had the bug that if I moved the mouse too fast upwards, scrolling wouldn't start at all.

It would be great if kitty could continue scrolling even if the mouse is dragged into and beyond the window decoration.

[nh2]

Thanks for the fix!