Details

• Browser and browser version: Chrome 89
• OS version: macOS catalina
• xterm.js version: At commit 376b296

Steps to reproduce

Happened to me using the demo with zsh with powerlevel10k
Comparing canvas vs webgl with ligatures

Canvas

Webgl

Below, I'm using suggestions and syntax highlighting plugins for zsh
It's suggesting === (untyped part of the suggestion should be in gray)

On typing =
Both

On typing ==
Canvas

Webgl

On typing ===
Canvas

Webgl

Looks like remnants of earlier paint are there in webgl