An old comment in the ProjectManagerView ctor mentions a horizontal scrollbar, something I cannot remember having seen in the KDevelop5 project manager, and that I've sorely missed.
This patch restores that feature, doing away with elided filename representation. The crucial change is setting setStretchLastSection(false) and the addition of a resizeEvent filter that sets the header's minimum section size appropriately. This was copied from Qt Creator's navigator widget implementation.
The tooltip feature showing the full filename remains functional so I think (and hope) there is no need to provide a UI configuration option to allow users to select either the new or the old behaviour.