git clone https://github.com/phx/signal-desktop-lockscreen
cd signal-desktop-lockscreen
./signal_lockscreen_patcher.sh
Run this script to patch your Signal Desktop installation to support an application lockscreen, and activate the lockscreen with Ctrl + L.
It runs 5 seconds after Signal starts in order to allow your messages to load, and can later be invoked again like above by the Ctrl + L keyboard shortcut.
This interactive script will prompt you for a password that will be stored in your Signal configuration directory.
You will need to run this patch every time Signal Desktop is updated in order to keep it active.
I have already submitted Pull request #4439 to the Signal development branch, which implements a pseudo lockscreen functionality by locking with Ctrl + L and unlocking with either Ctrl + ; or Ctrl + '.
The reason I have not submitted this as an official pull request is because it requires more serverside interaction than I am familiar with and would have to be implemented in Node.js rather than clientside JavaScript. It's a great functionality and works perfectly for my use cases. Please inspect the code before running it in order to assure yourself that it's not doing anything malicious, as Signal is a very sensitive and private application that we want to keep sensitive and private.
If Pull request #4439 ever gets approved and merged into the official Signal Desktop master branch, I have made sure that this patch continues to work when applied. Instead of having a pseudo-lockscreen with Ctrl + L, you will have an actual functioning lockscreen with password integration, which this patch provides.