I have been having a persistent though intermittent bug in Gmail where after command-tabbing back to Wavebox, the cursor is aligned correctly with its target; specifically, the email highlighted is a row below the position of the cursor. When I click to go to the email (or mailbox), Wavebox treats that as a click and hold. When I move the cursor into the labels pane and up out of the window frame and then back, this seems to fix the problem (temporarily).
The installed extensions I have are Slack++, Dropbox, and Dark Reader. I had the same problem on 10.13 with another laptop and only had the Dropbox extension installed.
Video below demonstrates the problem.

Thanks for reporting and for the video. I've managed to reproduce this here - although it does take a little while for it to kick in. Out of curiosity, roughly how long would you say it takes for the behaviour to appear for you?
Would you be able to try a couple of things your end, hopefully it will help me get to the root cause of it...
Account Settings. Under the tools section, double click Clear all browsing data. You'll need to sign back into the account, but does this fix it at all?It seems like it takes 10 minutes or so away from Wavebox before the issue crops up.
Resizing the window seems to fix the problem during that interaction with the app.
I'm working on trying the other things you suggested 馃槃
Having cleared all browsing data, the problem still comes back
Disabling dropbox also doesn't fix it.
Thanks for trying those! I did suspect that disabling dropbox would fix it, but it looks like I'm barking up the wrong tree with that 馃. Maybe the setup I'm trying it with is different here...
A few other things to try changing, which might help...
Sandboxing and enable Isolate Account Processes to see if that makes any differenceThanks!
Ok, so disabling all the extensions seems to have helped and the problem went away.
I don't have gmail offline or other gmail add-ons
I haven't tried the sandboxing.
These tests were run on a loaner macbook air while my normal macbook pro was being repaired. I now have it back with a fresh install of Mojave and Wavebox and all seems fine. I installed the dropbox plugin two days ago and the problem hasn't come up but I'll mention it if it does.
That's great that it's working on your new machine. Would you be able to share the About this mac screen for each machine? I wonder if a different graphics card has something to do with this :-/
Seeing that you're not seeing this anymore, I'll close this off. If it comes up again though, just recomment and I'll take another look. Thanks for taking the time to report!