Pointer stuck on drag mode

I've been using the same Mac hardware for about five years. Over the last two weeks or so, my mouse pointer has become stuck on "drag" mode about four times. Clicking works fine, but if I click and drag a file in the Finder or an object in Illustrator, it won't let go until I hit the Escape key two times.

I don't have Mouse Keys enabled in my Accessibility preferences. I don't have a trackpad.

It's not a problem with the mouse or any one application. I can unplug/replug the mouse, replace the mouse, quit all applications, relaunch the Finder, etc. but the problem persists until I completely log out and back in again, or reboot the computer.

27" iMac (2010)

OS X 10.10.5 (Yosemite)

Wired Logitech optical mouse