Power BI Desktop Version: 2.42.4611.701 64-bit (January 2017) Installed today.
Login window blocked by "Working on it" spinner popup after Power BI Desktop idle in background for a bit. I didn't nail down the time but both times it was over 20 minutes. The login expires after an indeterminate amount of time of inactivity, over 20 minutes at least. Clicking publish brings up the login window but is behind the "Working on it..." popup.
I have to end the task, open Power BI, click login up in the top right corner, then I can publish.