When using Trix on a narrow screen such as an iPhone 5 the toolbar does not wrap over multiple lines. I can see in previous tickets (https://github.com/basecamp/trix/issues/140) that the toolbar was on multiple lines but when using the current version the toolbar does not wrap.

You can set flex-wrap: wrap; on .trix-button-row in your application's styles if you want wrapping.
https://github.com/basecamp/trix/blob/01c2e7b345a53b869d9aa86b8daf1811f7b21f21/assets/trix/stylesheets/toolbar.scss#L14-L18
I applied @javan's solution by adding the following to my site's CSS:
trix-toolbar .trix-button-row {
flex-wrap: wrap;
}
Most helpful comment
You can set
flex-wrap: wrap;on.trix-button-rowin your application's styles if you want wrapping.https://github.com/basecamp/trix/blob/01c2e7b345a53b869d9aa86b8daf1811f7b21f21/assets/trix/stylesheets/toolbar.scss#L14-L18