Currently, this is indicated by the status bar of the active window being printed in bold text, which can be difficult to notice. So, it's hard to discover, for example, what causes j and k keys to sometimes to do the same thing as Up/Down and sometimes not, and how to intentionally affect this.

I would suggest adding an inactive-cursor color label, and using it for the cursor in the inactive pane. I think I might be able to implement this, if it is desired (both in the abstract and in terms of people's energy to review prs)

We could also optionally (or later) change the default of inactive-cursor to be default default reversed, especially if #1279 was fixed (though, after looking at it a bit, I'm confused about how). Another possible value is to print an underline instead of changing the default.

Another approach would be to allow changing the background color of inactive panes. Perhaps there are yet others.

Update: There's also a minor bug that might not be worth a bug report: if you use the mouse to switch the active window, the old window's status bar does not stop being bold.