Why does "some button was clicked" show up? Interferes with some apps

[AC Question]

My nexus 6P after the March security update was installed yesterday, started showing the "some button was clicked" message at the bottom. It does nothing I can see, but it does interfere with some apps and closes them. I have tried restarting and turning on and off the developer options, but nothing seems to make it go away. Anyone else have this issue?

 

[phatmista]

Commenting because this is my question.