Earlier quoted context omitted.
`flex-wrap: wrap;` and maybe, if you want, a `min-width` on the boxes themselves
Wrapping is undesired, the linear presentation is critical here. And the box is the screen in this case, on a mobile one the highest order buttons are missing. It’s sad that this had been up for so long with no real solutions. A genuine failure of the CSSWG.
You can't have >x of reasonably sized and spaced boxes on a screen horizontally without wrapping, regardless of layout engine. Besides, wrapping can be done presentationally while keeping the linearity.
In this case, I'd probably switch to an automatic grid layout with equal size boxes regardless of content size. Then it could neatly wrap into multiple rows while keeping the boxes at a constrained size.