Mouse mysteriously stops working on my right-hand screen

So I'm running Ubuntu 14.0.4 on my (Dell Vostro) laptop, and have a second screen plugged in. I use a normal USB mouse.

Usually, it works fine, but today (after hours of normal usage), I'm having this weird behavior, which I've had once before in the past:

I can't click on anything on my laptop screen (that's on the right-hand side), except the side and top bars - I can move the mouse all over the place, but there is no mousover or click events on any window on the laptop screen (things work fine on the external monitor).

And if I click on the Dash search, I get the search results full page, and the mouse works fine there (there are reactions on mousover and I can click). So it's as if there was this invisible window running above all the others, absorbing all the clicks, but that can still be overlapped by dash like all windows can (nothing of the like appears if I alt-tab around).

And if I unplug my desktop screen, clicking works fine on the laptop (and goes away once I plug it back in). And if I open the system settings and move the (virtual) relative position of my screens around, the problem will only appear on the right-side screen, whether it's the laptop or the external monitor. If I put the two virtual screens one above or below the other, I can click everywhere just fine, until I put the screens back next to each other - so it does really seem to be the "right hand side" going wrong, not the laptop screen itself.

Last time this went away when I closed my session and reopened it (locking it is not enough). I don't really know what more I could do to troubleshoot this.

Edit: clicking suddenly started working again once I finished posting this. Despite the fact that I did nothing "special" (I clicked on a chrome gmail window with a notification?), and had tried dozens of things while writing the problem report.

Solution 1:

I had the same problem [Xenial], which seemed to be triggered by starting VirtualBox (though that could be a red herring). I fixed it by

unity --replace

Though that doesn't explain why it happened in the first place...

WARNING - this will close all your open applications, so save everything first!