Navigator - Window1 has red flag

I’m working on a desktop project and today I noticed for the first time that the Window1 icon in the Navigator has a red marking in the upper left-hand corner. I never noticed any red markings in any of my other projects. Does this have a particular meaning and if so, how do I address it. I’m looking in the reference materials but haven’t stumbled on anything yet.

Are you using a beta release ?
If so you should move this to the pre-release channel

I am using Version 2019 Release 1.1 on a Powerbook running macOS Mojave Version 10.14.6

Powerbook ???

Did you happen to add some attributes to the window ?
That might be a tiny little letter A

I meant MacBook Pro (long morning!). It does appear that I inadvertently/unknowingly added an attribute somehow. When I deleted it, the flag went away. Thanks.