Left-click in piano roll brings up contextual menu

This has been driving me nuts and I'm hoping somebody has an answer...

In the piano roll, simply left-clicking in the window (not on a note) keeps bringing up the contextual menu as if I'm right-clicking...

I'm a long-time Logic user and I've always dragged the mouse around inside the window to select multiple notes, and now I can't do this simple function because the menu keeps popping up.

At first I thought this was a weird change that was made in Logic X, because I noticed that the manual talks about selecting multiple notes by clicking on the keyboard on the left, but that doesn't make any sense because you need to select certain areas far more often than all the same notes in the region.

But then one day, I was able to click inside the window just like I have been able to for all those years. This makes me wonder if there's some new preference setting that I accidentally turned on or off, or if this is simply a bug that happens sporadically.

Has anybody else come across this problem?