It's been happening since yesterday in version 2.78b, and I have no idea what I might have accidentally clicked in any of the menus that could have caused this to happen. If I move, rotate, resize, or a few other actions, as soon as I click my left mouse button to stop the action, it starts rendering. I have checked User Preferences, and rendering is set to F12, so that's not it.

Any hints, or suggestions, would be greatly appreciated.

You sure it's 2.98b? Is that a daily build? That's like 10 versions ahead of the latest stable release.

I see you changed that. Ok, sorry. I was going to say if it was a daily build then it must be a bug.

BTW, Miss B, I can't be sure this is the issue, but if you have Auto Render turned on by accident it could cause an issue similar to this. Make sure the Auto Render select box is NOT checked at the panel on the bottom of the screen.

Nevermind the previous suggestion because you need to be in the compositor window to see that option anyway, so it probably would not be causing your issue unless you were in the Compositor window at one time or another.

Hmmmm I'll check into that in any case. I'm sure I hadn't planned on being the Compositor window, but I could've accidentally entered it, selected the Auto Render, and didn't realize it.

Thanks for the suggestion in any case LuxX, because it sounds plausible, and I can't think of anything else that would cause this.

LuXxeon you are a genius!! I just opened the scene again, and sure enough Auto Render was checked. I don't do animations, so my bottom "window" is the Node Editor, and sure enough, I must've accidentally checked it and not realized it. Now I'll make a note of this, in case it happens again.