Oh boy. https://github.com/signalapp/Signal-Desktop/issues/1635
tl;dr Signal Desktop is based on Electron, which in turn is based on Chromium 58-59, and it seems to be affected by bugs that have been fixed in Chrome/Chromium 60-62.
Gotta love #Electron. As somebody said "now everyone is running 5 different instances of old insecure versions of the most scrutinized and attacked application on Earth."