System information (version)

• OpenCV => 4.2
• Operating System / Platform => Ubuntu 18.04
• Compiler => clang++-6

Detailed description

In a test program where I was using ALT + Left click to run a special mode that's different from just Left click, I was seeing weird behaviors before I tracked down that my NUM_LOCK was on. Apparently, anytime my NUM LOCK is engaged, the ALTKEY event flag (32) get set high on opencv window event callbacks.