I just installed plsql developer 15 to test.
I almost exclusively in full screen mode to see more lines of code, in addition to have more lines visible I have deselected sdi in view menu. However now, whenever I switch to full screen mode, it seems it switches to SDI despite having it disabled in the menu. When I disable it again, it does not stay in full screen mode, instead goes back.

This SDI mode adds another line with tabs of all open windows on top, which I do not need as I use the window list.
how can I either disable sdi or get rid of this tab line, to have more lines of visible code?

SDI mode and maximized window mode are now essentially the same. If you go to Preferences > User Interface > Settings for Single Document Interface, you can select the "Don't show tabs" option to get more space and a more standard maximized window mode.

thanks, that setting helped

almost the same, but the full path in the window title is lost, as this is a setting for MDI only, can this be changed?

I have added this to the list of enhancement requests.