So please at least add a description to it (and the LuaStyle) to indicate that it's the progress bar's "thickness" instead.
I only figured it out after looking at the base game's code comments. I spent too long not looking at the base progress bar style because I was trying to make it mimic another element so I could use it's runtime re-colorability and could not figure out why it wasn't displaying the shadow or bottom gradient.
ProgressBarStyleSpecification::bar_width is confusingly named
Re: ProgressBarStyleSpecification::bar_width is confusingly named
Confusing indeed, description added to the docs for the next version.
I'm an admin over at https://wiki.factorio.com. Feel free to contact me if there's anything wrong (or right) with it.

