As a general rule, when you rearrange any of the windows / panels in MPLAB® X IDE, they will stay where you put them the next time you open the IDE. Unfortunately the Dashboard window doesn't share that behavior due to the way other windows load.
As an example, the most common place to move the Dashboard is to the bottom left with the Navigator window. However, the Navigator window and its container panel only open once other events take place, such as activating a source file in the editor. As such, when you first open the IDE or a new project, the lower left panel hasn't yet been initialized and there is nowhere for the Dashboard to dock into. So, it does the next best thing and docks itself into the same panel as the Projects and Files windows, since they are the first to open.
While it may be possible to fix this behavior in the future, the fix apparently isn't as trivial as it might seem due to the Navigator being part of the NetBeans core and the Dashboard being part of the MPLAB X IDE specific code.