Moving focus between panes doesn't always work #3045
Description
Messing around with panes today, and found it wasn't walways possible to move focus correctly. This was the setup:
I believe the tree was like this:
+--------------------------------------------------------------+
| +---------------------------+ +----------------------------+ |
| | +----------+ +----------+ | | +------------------------+ | |
| | | | | +------+ | | | | | | |
| | | | | | | | | | | | | |
| | | | | | 2 | | | | | 4 | | |
| | | | | | | | | | | | | |
| | | 1 | | +------+ | | | | | | |
| | | | | | | | +------------------------+ | |
| | | | | +------+ | | | +------------------------+ | |
| | | | | | | | | | | +--------------------+ | | |
| | | | | | | | | | | | 5 | | | |
| | | | | | 3 | | | | | +--------------------+ | | |
| | | | | | | | | | | +--------------------+ | | |
| | | | | | | | | | | | 6 | | | |
| | | | | +------+ | | | | +--------------------+ | | |
| | +----------+ +----------+ | | +------------------------+ | |
| +---------------------------+ +----------------------------+ |
+--------------------------------------------------------------+
Actual
Moving from (2,3) -> 4 was NOT possible.
Expected
Moving from (2,3) -> 4 was possible.