Login password entered in clear text to Slack while at the login screen

I just upgraded my Macbook Pro to Mojave. Upon reboot, I was at the login screen. I entered my password and hit return. Login did not occur. The textbox for password entry had not been selected. I clicked on the box, entered my password and hit return, logging me on. Slack was open in the background and my login password had been entered into the last Slack thread I had been in. I found this rather shocking since it seems like a big security flaw if a program can capture text from the login screen.