I have a brand new iMac running High Sierra 10.13.4. I have installed Minecraft 1.12.2 and Forge installed from forge-1.12.2-14.23.2.2654-installer.jar downloaded yesterday. When I launch Minecraft in fullscreen mode, everything looks normal. When I launch Minecraft with Forge in windowed mode, everything looks normal. When I launch Minecraft with Forge in fullscreen mode, I see this: (my pointer is over where the Quit Game button would normally be in fullscreen)
When I go into a world, the black part of the screen goes white, but the game is still just in the lower left corner and the mouse location is offset.
I searched for this issue and found some old recommendations to modify splash.properties which doesn't exist and add -Dforge.forceNoStencil=true to the JVM options which had no noticeable effect. I have tried removing, redownloading forge, and reinstalling. I have tried deleting the entire minecraft folder and reinstalling everything.
When I choose the launch option without Forge, everything is still normal. This only happens when I launch with Forge.
forge-1.12.2-14.23.2.2654-installer.jar.log
debug.log from minecraft/logs
latest.log from minecraft/logs