The last update to the UI changed the “X” to a left-pointing arrow, suggesting that it navigates backwards in the pane’s content rather than closes the pane. That’s good!
But now that leaves us with pane-controls with two left-facing arrows, like this:
It’s not clear to me what one versus the other does, and I still have to stop and think every time.
My suggestion for improving this: change all the “collapse” arrows (which is currently either left- or right- facing, depending on the pane) into thick minus signs ( ), and all the “expand” arrows into plus signs ( ).