The docs of Set FOCUS=toggle say:
Toggles the input focus between the left and right file displays.
The current behavior is (testable with command in toolbar button):
- With a file display focused, the other one is focused, as expected.
- With any other control focused (like a tree, or the script log), the left file display is always focused, no matter whether it's source or destination.
Other approaches when a control other than a file display is focused are:
- Do nothing.
- Focus the source file display.
- When a tree is focused, find out which file display it's currently associated with, and focus the other file display. When neither a file display nor a tree is focused, focus the destination file display.