Sideways tabs are used widely in KDeveloop. This is an usability issue. People read from left to right, or maybe right to left, but not from bottom to top with rotated text. The user should be able to disable totally vertical text in the IDE. This option can't be disabled inside the MDI windows.
Suggestions on how to fix?
I don't know why vertical tabs were used in the first place - maybe in order to save vertical space - but the title is duplicated, hence there are no savings. So why not use regular tabs ? I know these dockable windows can be hidden by clicking on the title, so it's a little more than tabs.
I'm also not a fan of vertical text. A good solution would be to have an option to only show the icons in the docks. That way we can also keep more panels in the left/right docks. Something like like Atom's toolbar plugin: https://atom.io/packages/tool-bar
We could do it like QtCreator, I guess. I personally don't see an issue, I think it's readable just fine. It's not like you would struggle to decipher the word "Projects" written on the tab fifteen times every day.