Created attachment 114038 [details] folder list KMail is adding empty columns in several places like folder list, account tab in the settings. The columns can't be removed. Please see the screenshot linked. Plasma 5.12.6 Frameworks 5.47.0 Linux 4.17.7-200.fc28.x86_64 Fedora 28 Kmail 17.12.3-1.fc28
Created attachment 114039 [details] account setting
A change in Qt 5.11. It now respects the minimumSectionSize() in QHeaderView, so it needs to be setup correctly.
Seems to be fixed for me.