UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.77 Safari/537.36

Steps to reproduce the problem:
1. make web app for mobile that goes fullscreen
2. open in dev tools and run on localhost
3. app calls fullscreen

What is the expected behavior?
I want a nice way to silently prevent fullscreen when I am developing an app targeted at mobile, on desktop; or just apply the fullscreen to the app in the mobile emulator.

What went wrong?
the app goes fullscreen, which is irritating since I lose access to my browser tabs, etc ... it is *always* the case that my very next action is to cancel the fullscreen (using the FAB that drops down when you move the cursor to the top)

Did this work before? No 

Chrome version: 70.0.3538.77  Channel: stable
OS Version: 
Flash Version: 

I added code to my app that tests the window.location.host for 'localhost' and doesn't attempt fullscreen if it is localhost. Unfortunately, that has just bitten me because it produces a different code path, and so different behaviour.
What I really want is some way to disable it while I'm in devtools. A little checkbox, like the 'disable cache' button would do it.
In any case, since I am developing for mobile, I am using the device emulator, and it seems to me that, if anything, the 'fullscreen' call should apply to the app inside the emulator, and not to the browser itself.