Fix split pane divider hovering over neighboring split pane

[dmaluka]

Fix the following funny issue: if we open 3 vertical split panes (i.e. with 2 vertical dividers between them) and drag the rightmost divider to the left (for resizing the middle and the rightmost split panes), it does not stop at the leftmost divider but jumps over it and then hovers over the leftmost split pane. And likewise with horizontal split panes.

[dustdfg]

I am not sure if it is critical but this commit makes impossible to fully hide the leftmost split. It will have at least size of one character

[dmaluka]

Yeah, right. Maybe it is not critical per se, but it would be good to understand what exactly is going on. At the moment I don't understand why size <= 0 prevents fully hiding the leftmost split but does not prevent fully hiding other splits. (Or in other words, why is size < 0 check not enough for preventing coalescing split panes, so that I had to use <= instead of <.)

Also, this issue might be just a tip of the deeper issue #3071.

[dustdfg]

I have a small guess related to one of my issues #3046 about wrong resize logic. Since resize logic can work wrong anything that relies on correct work can accidentally brake up