Hello @AmandaCameron, please copy your system information from steam (Steam -> Help -> System Information) and put it in a gist, then include a link to the gist in this issue report. Additionally, what model mouse are you using?
Appologies:
The issue happens with both the UHURU Bluetooth Mouse as well as the touch-pad built into my laptop.
There's no particular reason to track this separately in the wine issue tracker instead of the Proton issue tracker, so I've transferred this issue to https://github.com/ValveSoftware/Proton/issues/1418.
As mentioned in This comment for Subnautica there's some mouse issues where there's unexpected movement during mouse button presses. I'm experiencing a very similar issue in Stationeers (APPID: 544550) so I suspect it's got something to do with unity3d, which both games use.