The bug

When you open a singleplayer world 0% can appear for a short moment of time. This value never increases and there is no title or additional information.

Code analysis

Based on 1.11 decompiled using MCP 9.35 rc1

This happens because the method net.minecraft.client.Minecraft.launchIntegratedServer(String, String, WorldSettings) displays a new net.minecraft.client.gui.GuiScreenWorking.GuiScreenWorking before sending the login packets to the internal server. This is probably done to prevent the world selection from being displayed after the packets are sent. This would happen without this GuiScreenWorking because it looks like the loading screen is only a overlay and not a proper GUI (net.minecraft.client.Minecraft.loadingScreen).

It would probably be a good idea to replace net.minecraft.client.LoadingScreenRenderer with a proper GUI.