Did setup a Raspberry Pi which shows the openHAB HABpanel. Had to figure out how to start the browser in fullscreen mode right after the Pi starts.
Blogged about it here: https://andreas.scherbaum.la/blog/archives/992-Automatically-start-a-fullscreen-browser-on-a-Raspberry-Pi-show-openHAB-HABpanel.html
I did the same with FullPageOS: https://github.com/guysoft/FullPageOS
Do you have the the experimental GL desktop graphics drivers enabled?
Which Raspberry Pi did you use?
I did use a Pi 3B (non-plus)
I assume you have display/monitor connected directly to Pi. Why you go this way?
My first pi project had touch display connected as well. For openhab I decided to be display independent. So I have still my Pi with Openhabian 2.5 and connect with HABPanel over http, with any display, monitor I could.
I bough Fire HD for ca. 74€ and have panel for my automation system as well as tablet in one whenever I wish. Consider this way. It is more independent and less expensive.
Unless you have other objectives 
I have multiple Pis:
I did choose this way, cuz I wanted to test the official panel. I am totally in control of this device, the performance is good. I think it will be supported (ie get updates) for years. I won’t have any of these advantages with any tablet.
I have multiple unused Pis and microSD Cards. So my current investment was also only 75€ (display).
I’m using a Pi 3+ as well.
Unless the experimental drivers are enabled by default, they are off.
I installed a Raspbian on it, did some basic configuration, but mostly focused on getting the display up and running.
There is only the 7" (official) touchscreen connected, no extra monitor.
That is supposed to be a small display which we have in the kitchen, but over time it turns out that the compination of openHAB and Pi and touchscreen is not really reliable. I’m hunting more issues than I want to spend time on.