In MobileFrontend we set a global $wgMFDeviceWidthTablet and make this available in LESS and js config. It basically defines the threshold where we consider a mobile device to be a tablet device. Note we do not define a desktop threshold (which we should do)
Flow has $wgFlowDeviceWidthTablet
I've just added this to Extension:Gather
In core the magic 768 number appears in mediawiki.action.view.categoryPage.less
Time to make this variable in core?