— what phone model are you testing on? When i still had the Samsung Touchwiz UI installed on my phone i had the same issue.
also did you make sure fullscreen is selected in XDK Project settings?
Tested on Nexus 5, 7 and LG G-Pad 8.3.
Ofc fullscreen is enabled in XDK.
I also notice if the buttons are visible, if I go to home and re-open it again, it opens in immersive mode.
It looks like Xwalk is set to call immersive mode on launch once, or on resume once. If anytime the buttons return for whatever reason, it won't go back to immersive.
Also, once the app is closed (via Browser => close on back button), the next launch is not in immersive mode. You have to clear the memory then launch and it works.
For those curious, cranberrygame's Phonegap IAP plugin works excellent! It's literally instant verification, unlike with CocoonJS where there is a few second delay.