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?