Testing with 5.5.3595 (latest download) with Visual Studio 2019 16.5.3, I’ve hit a persistent issue.
Something makes auto-complete get into a state where closing the auto-complete window de-focuses Visual Studio (like hitting alt-tab). It doesn’t do this all the time, but once it starts it’s a struggle to get rid of it (restart VS sometimes fixes it, but not always?) and almost impossible to type with. I suspect trying to do things before VGDB has finished loading a project might make it more common but haven’t been able to prove it yet and its not the only cause if it is.
Strange, we haven’t changed the IntelliSense popup logic recently, so if it didn’t happen before, it’s likely caused by some other change on your machine (e.g. installing another extension, or some software using global window hooks). If you could narrow it down to something we could reproduce on our side, we would be happy to fix it, but otherwise it’s unfortunately impossible to tell what could be causing it.