In nightly ...g158f2ec from Dec18 I found the left-right-on-off choice in the dockwidgets-tab quite useful.
In my layout it allowed me to get rid of the upper same task longitudinal bar, which would cover parts of the main tab if I went to fullscreen (F11).
Also I could not move it into the vertical columns left-hand as they are widened unnecessarily.
By the way, "fixed extensionline length" still doesn't work any more. Or is it only in my installation somehow?