In the "Sidebar > Navigation" settings, I select the "Only button" to have a lean and compact bar.
The problem is that the text of my longest image folder name gets truncated; see "Informations pratiques" in the attached screenshot, the final "s" is missing.
It looks like you have set a maximum default menu width, right?
This make sense when you don't use the "Only button" setting, as this would be displayed permanently on the screen. But with the "Only button" setting, the length could be set to the maximum folder name length without problem, as this is a transient display.
Another solution to solve this issue, would be to add a user parameter, that enable to override the default width.