'ontouchstart' in window return true if TOUCH_EVENTS is enabled, which is the case in the GTK port by default. That should probably return false if no touchscreens or touch devices are present, even if TOUCH_EVENTS support was compiled in. See https://bugs.webkit.org/show_bug.cgi?id=142888 which fixed this on Mac (not ios) where TOUCH_EVENTS is disabled in OptionsMac.cmake. This bug causes seemore.playcanvas.com to return touchscreen controls when clicking on the 'gamepad' icon on the top right.
*** This bug has been marked as a duplicate of bug 139681 ***