This is a forge port of Seamless Loading Screen by Minenash!
Seamless Loading Screen is a client-side mod that takes a screenshot of the game when you leave a world or server and displays it when you rejoin it. Screenshots are saved within a subfolder under your screenshots directory.
A video demonstration (from the original creator):
Sodium compatibility: If you have Halogen or Magnesium (or any other sodium ports) installed, add the line mixin.features.gui.fast_loading_screen=false into the sodium-mixins.properties config file. Sodium overwrites the loading world screen which this mod mixins into for the transparent background; adding the line fixes Minecraft from crashing at launch.