STR: https://firefox-devtools-tour.glitch.me/demos/test-case-flex-width/

Refresh a few times.

The top and lower input fields (incl buttons) 100% of the element, but the on some refreshs tha upper one ends up overflowing. I could not reproduce this in Chrome.

I always see the overflow, fwiw.

I always see the overflow, too.

Reduced testcase for the overflow:

data:text/html,<div style="display:flex;float:left;border: 5px solid red"><input type="number" style="flex-basis:1px">

This is a version of bug 1316534, and the "explicit" size here is in forms.css:

input[type="number"] {
  [...]
  inline-size: 20ch;

https://searchfox.org/mozilla-central/rev/153feabebc2d13bb4c29ef8adf104ec1ebd246ae/layout/style/res/forms.css#981,984

(This is an especially weird case of bug 1316534 because (a) the "specified size" is applied by a UA stylesheet rule, and (b) that specified size is actually lower than the min-content size, apparently.)