Yes, unfortunately, GameMaker’s capabilities of doing “full screen” in the html5 runtime are somewhat limited. i need to resize by hand when i detect window size change. and as everybody has different height of windows taskbar or browser-tabs (favorite bar or not) i am a bit limited in resizing when i want to keep the aspect ratio.
If you play on mobile, the button clicks need to be launched via double-tap. not sure about pad-tapping, i don’t own one