Make the properties panel open and closable. This is more a product feature. 
Use Case
A plugin developer is creating a video with XD one side of the screen and the exported renditions in the other side of the screen. To show more of the artboard he would like to be able to close the properties panel.
Then, however, it really doesn’t belong to the “API Feedback” category (even if you provide a plugin-related use-case – as you’ve stated yourself 
).
This forum is meant to be about plugin development only, so this belongs more on UserVoice (don’t get me wrong here: I’m not making those rules – I just want to stop a great discussion from starting which will probably lead to nothing since it’s not the right forum 
).
See @ashryan’s comment on Horizontal menubar for Windows 10 XD app - #5
(that thread got closed afterwards)
Related UserVoice sections:
That is why I caveat. 
There has been a discussions where it’s been stated if it’s plugin related it’s ok to post and I remember hearing it was talked about but I couldn’t find it on this forum. In this case I wanted to give a use case whether it is used or not then the plugin team can say, “We also have use cases for this feature.” Thanks for the links
