Dan, It turned out to be an unneeded View.invalidate statement.
Once it was removed, all was OK.

I don't have any UI.start_timer blocks so that can't be the problem. I did copy the entire code statement "UI.messagebox("There are no valid snap points", MB_OK)" to the console and the message displayed normally. I even added a UI.inputbox statement to see if it would display and, like messagebox, the tone sounded but nothing displayed. Other than a method that does a calculation, I can't think of anything in this plugin that I have used many times before.

In the middle of testing out a new plugin I was creating, the UI.messagebox stopped displaying. The command was being executed because I could hear the beep and the plugin would lock up until I pressed Enter. I added a UI.messagebox command in a reset method that is executed when plugin starts and that one displayed as expected but no other messageboxes after that.