If you remove that width rule, or overwrite it with a rule targeting .textbox header and setting the width to 100% or auto, things should be back to ‘normal’.
Thanks Reece. You’re quite right – this sounds like a problem with a legacy theme (i.e. Baker). I’ll file a bug report and we’ll fix in a future deploy. In the meantime, you can add this rule to your web custom styles:
You’re welcome. I think what happened here is that the Baker theme originally had some CSS in the webbook that made section header text 50% width (that’s what gives the chapter titles their half-page effect in webbooks). It’s an older theme, and when we added the ability to make additional textboxes (which also have little header elements in them), we didn’t catch that the CSS rule for the webbook set their width to 50%. It’s easy enough for us to fix, and a patch will come out on in our next release. If you encounter other bugs in the future, please consider reporting them here: https://pressbooks.com/report-a-bug/