Welcome Screen frequently shows "Welcome to Tails!" is not responding

In test suite run https://jenkins.tails.boum.org/job/test_Tails_ISO_20233-fix-onioncircuits/2, three tests failed because the dialog appeared in the wrong moment and ate the keyboard input that should have gone to the "Start Tails" button.

The reason seems to be that we set the timeout to 300 as a fix for #20227 (closed).. We chose that number assuming that it meant 300s which would be enough that it should never be reached in practice. Turns out that's actually milliseconds, so we set it from the default 5000 (5s) down to 300 (0.3s).