I am very new to Node-Red, so I could be wrong here 
but based on what I have seen and read about, the default dashboard is limited in customizability in favor of relative ease of use.
I have found the the relationship of the dashboard Groups (I think the Tabs are what is inside the Groups) is based dynamically on the window size... thus stacked vertically on mobile, and horizontally then vertically on larger screens. This can be demonstrated by switching between full screen and elongated windowed view of same dashboard...
