Hi,
thanks for this report.
It comes from the nature of the flex-basis property that works differently from width/height. In the meanwhile you can probably work around this issue by applying overflow: hidden to the data-simplebar element or just use width instead of flex-basis

I had a deeper look at this and the solution is not an easy one.
It's definitely fixable but it involves quite some code so I will wait for now for more reports. If this is really blocking (as in, the workaround I proposed can't solve that), I will try to get a fix built-in.

Thank you.
I am currently using a temporary method to use width instead of flex-basis

I have the same bug with infinite 'width', when I used

<section style="display: flex">
<input style="width:100%"></select>
</section>
<section style="display: flex">
<textarea style="width:100%"></textarea>
</section>

Infinite width appears only in the mobile view (when the user-agent changes for mobile devices) and in MacOs browsers. I see errors in the console "resizeobserver loop limit exceeded".
Also simplebar calculates the wrong height of that and i can't scroll to the end. It reproduce only in mobile view. And I don't know how to fix second bug :(

Guys, I just released simplebar@4.0.0-alpha.0 that would be awesome if you could let me know if this is still happening with the new version.
Thanks for your contribution!

I have trying with simplebar@4.0.0-alpha.0, but problem is still continuing.

Ok thanks. It happens only when the scrollbar is floating. Got it.
I'll look into it.

I'm releasing simplebar@4.0.0-alpha.1 in a few min. Let me know how it's going.

All right, now the problem is solved.
Thank you for your wonderful project and hard work.