Sometimes switching to a different window tab lets you resize the pane. More info here: https://developer.apple.com/forums/thread/669955?answerId=654713022#654713022

I came here looking for an answer to why the navigation and inspector pane behavior changed, and how I could change it back. Besides the difficulty in resizing the panes, you may notice that all window tabs now share the same state and size of the navigation and inspector panes. That is, if you close a pane, and switch to another window tab, that pane will remain closed. If you open it, and switch back, the pane is now open in the first tab also. The same goes for the size of the pane. If you can't resize it, switching to another tab sometimes lets you resize it. That new size will stick when switching back to the first tab. You may also note the contents and expansion state of the panes are still independent of the window tab you are on, which makes this change even more puzzling. I find this new behavior very annoying, and counterproductive. I have different panes open on different tabs for a reason. For example, window tabs with storyboard or nib files do not show the navigation pane, but do show the inspectors, while tabs with code files are the opposite. The only workaround I have found is to use a separate window altogether for my IB work. I do know the keyboard shortcuts for hiding/showing the panes, but even having to do that is a pain (painful pun intended).