The bug

It seems that whenever the game shows the text of an hoverEvent (caused by /tellraw, hovering over an item in the inventory...) it changes the lighting of entities.

Affected entities

• Displayed block of minecarts (darkens)
• Ender crystal (brightens)

How to reproduce

1. Place a minecart displaying a block:
   Example command

   /summon minecart ~ ~1 ~ {DisplayTile:"minecraft:quartz_block",CustomDisplayTile:1b,DisplayOffset:16}
   

2. Stand in front of it (South direction; +Z) and look at it (looking North; -Z)
3. Use the following command and hover over the text:
   Example command

   /tellraw @a {"text":"","extra":[{"text":"Hover over this text","color":"white","hoverEvent":{"action":"show_text","value":"You are hovering over this text"}}]}
   

   → You should see that the block being displayed darkens when you are hovering over the message.

Code analysis

The following is based on a decompiled version of Minecraft 1.9 using MCP 9.24 beta.

It looks like the method net.minecraft.client.gui.GuiScreen.drawHoveringText(List<String>, int, int) which calls the method net.minecraft.client.renderer.GlStateManager.enableRescaleNormal() is causing this.