I suspect this is the same in vim as well, but fwiw I'm actually using nvim.
For a long time I've used
set mouse=a so I can choose to navigate my file with the mouse, if I so wish.
What I've never noticed today is that if I click into PuTTY with nvim open, is that this has the side effect of automatically entering visual mode. Normally I don't click on PuTTY very much, but just alt-tab to it, but today I have a lot of windows open, and have connected two external screens, and I find myself clicking, and then having issues as I'm suddenly in visual mode when I don't expect to be.
If I remove
set mouse=a (or do
set mouse-=a) then this behaviour goes away, but I can no longer move the cursor within nvim with the mouse.
Any bright ideas?