I wanted to use a Unity WebGL build so that my game is easy to play, but after watching a few people play my game they ran into an issue where the mouse gets stuck like the above picture and becomes unusable.

(Yes, I took a picture of the screen, I didn't have handy a screenshot tool that included the mouse).

I found a fix from this Unity thread:

https://forum.unity.com/threads/web-gl-mouse-input-glitch-on-chrome.468362/

Copy-paste this line of code into the CSS file generated with your Unity WebGL build:

.webgl-content * {border: 0; margin: 0; padding: 0; -webkit-user-select: none; -khtml-user-select: none; -moz-user-select: none; -o-user-select: none; user-select: none; }

It fixes the issue and your mouse-based games should now work fine in the browser!