The bug

Underlines in chat and /title command intersect each other, causing an ugly effect when fading.

How to reproduce

Take a screenshot or enter the following command:

/title @a title {"text":"words","underlined":true}

→ Note how the pixels in the underline between the individual characters of the word "words" are much bolder than they should be when the text is fading in and out.

Note: You can make the effect more obvious by first using the command

/title @a times 100 0 100

Code analysis

Based on 1.11 decompiled using MCP 9.35 rc1

The method net.minecraft.client.gui.FontRenderer.renderStringAtPos(String, boolean) has a strange condition when drawing underlined text. It enlarges the line one pixel to the left if it should draw underlined text, which is of course always true because it is currently drawing underlined text. Maybe it was supposed to test if the text is italic or unicode? But enlarging the line to the left means that it will overlap if a underlined character is in front of it, which causes this effect.

Removing this should probably not cause any problems.

Examples

Here are some examples: