Actually, the demo does use debug mode.
Okay I just found an even better way after I stumbled upon the API of
FlxG.keys.preventDefaultKeys = [LEFT, RIGHT, UP, DOWN, SPACE];
Better way without modifying your index.html, put this in your Main.hx or anywhere in your Haxe code:
#if js js.Browser.document.onkeydown = e -> e.preventDefault(); #end
This HaxeFlixel tutorial on masking could help.
I have actually tried blend modes myself. I used a transparent sprite, of course, but found no blend mode that uses the alpha of the top layer. Then I wondered if I could have a custom blend mode so I researched shaders as HaxeFlixel supports it. According to what I have read, shaders do not know nothing at all about other sprites. So, yeah, both didn't work, at least for me.
I think you gotta do that manually. HaxeFlixel only stops drawing off-screen FlxSprite.
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