Hi, I have all of my files stored on dropbox and I use the smart sync feature. My default approach is to keep everything stored online and not locally. Up until about a month ago when I moved to a new laptop, I was able to select a non local dropbox file, the file would be downloaded and it would appear in the viewer pane. Ever since I've moved to my new laptop, selecting a non local dropbox file will cause the viewer pane to freeze and a file preview never appears. The folder tree columns still operate normally during this time.
I would look at firewall and antivirus software/configuration as a potential difference between the two machines, if it was working on the old one but no longer does on the new one.
That should not affect things as far as I know, assuming the drive is either unlocked when you're doing things with Dropbox, or not involved with the Dropbox storage at all.
I did some testing and turned off bitlocker. No change in behavior.
I then did some further testing with onedrive and their version of smart sync. File previews in the viewer pane worked perfectly. I had always had plans to migrate from dropbox to onedrive, so this speeds up the timeline and solves the issue for me.
With no need for dropbox, I suppose it really doesn't matter to me anymore if the viewer pane works with dropbox or not. You can consider this issue resolved.