I've got a HTML5 game and I only want the on-screen controls to show on mobile, not on desktop.
Is there a built-in way to check if the user is on a mobile device?
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Use Platform Info object.
construct.net/en/make-games/manuals/construct-3/plugin-reference/platform-info
No, don't use the Platform Info object. See the example Detecting input method for the best way to do this.