In Focus Mode, enlarge top edge resize drag area

### Description of the new feature

I love focus mode for terminal windows. I just have a bit of a hard time resizing them from the top, for some reason the area where the top resize arrow for resizing up and down appears seems to be about 1px in height only. You really have to nail it.

For comparison, at the bottom edge of the window, it seem we have a bout 5px region where the vertical up/down drag/resize mouse cursor appears.

### Proposed technical implementation details

_No response_

Start by tracing Focus Mode's window-edge hit-testing and compare the top and bottom resize regions. Done means the top edge exposes a larger vertical resize target, comparable to the bottom edge, while the resize cursor and dragging behavior continue to work.

Written by the indexing model from the issue text.