Same issue for me. Also stuck on El Capitan (which I like so that's mostly okay). It is not an Electron issue but Joplin issue (well, okay, and El Cap issue), because I'm also using VS Code on this machine, and I don't have issues with that. Or else it quietly handles and recovers from the issue. Also, newest Evernote is compatible with Yosemite which is even older than my system, so again, not an Electron issue, probably. Anyway, same issue as Arkadi: when I launch, I just get a blank window, and I can't quit unless I go to Activity Monitor and force quit from there. But the app looked great and synced fine the first time I used it. So I'm hopeful 
Sorry to hijack. I did a search for Capitan (to find El Capitan issues) and saw the post from Mac user, Arkadi, so I thought I was in the right place. And the posts from Windows users read like the similar problem, so I did not realize it may be a different issue. Totally new here and also to discourse, so getting used to the format
Did anyone here ever find a solution for this issue? I have the exact same problem. After computer or monitor sleep/hibernation, joplin often becomes blank white, similar to how it happens when an update message pops up. I believe they might be related?