show-whitespace shows too many placeholders on Firefox with Minimum font size #3709

Unfortunately this is a Firefox bug.

We use the ch character to define how wide the "tab" image should be. Firefox does not adjust the size of the ch unit to match the actual character size.

Here's a barebones example: https://codepen.io/fregante/pen/rNLEqPr

<p>ciao</p>
<div></div>

html {
  font-family: monospace;
}
div {
  width: 4ch;
  height: 4ch;
  background: red
}

Regular on the left; minimum size 20 on the right

<img width="195" alt="" src="https://user-images.githubusercontent.com/1402241/99894508-eddf4800-2c49-11eb-91f0-247990cf8f43.png">

You can see that the word size increased, but the 4ch*4ch square doesn't follow it.