Concern:
The break should be a return, so that this function does not process further pending GUI messages that should be processed by the page that was just switched to (here session page).

Otherwise if there is a PlayerAssignments message processed in this function after processing a "start" message, then the session page will run with the outdated PlayerAssignments instead of the new ones, as reported in #5660.

This apparently is a strong race condition that happens so rarely that it was never triggered in 8.5 years, or there is a different undiscovered bug.