Hardware: Samsung Galaxy S3
OS: Android 4.2
Browser: Opera / Firefox
In Opera keyboard input switches to some weird mode with typed text on top in white input field and backspace doesn't work on already typed text, not sure if it's Opera's fault or you tried to make it more usable and it failed.
In Firefox keyboard arrows hide keyboard but i actually guess it's ff's fault, not yours, so it generally works acceptable. It's worth to mention that afaik Firefox does support fullscreen however doesn't provide any menu option to switch to it so it'd be nice if you could add soft button requesting HTML5 fullscreen for whole site. Also menus are quite unusable, they simply hide when I'm trying to access submenu.
Both browsers are zooming site to some sick levels i have 5mm high font and it's basically difficult to use as I need to constantly zoom out. I guess it should be fixed by zoom html tag ( maybe without user-scalable=no property) or eventually maybe this CSS rule disabling tap zoom could help.
Also such high zoom level messes up interface as i ugess your text editor (it's Ace isn't it?) doesn't support so small container. If browsers wouldn't zoom in/out site as mad it'd be even more or less usable even on phone like S3 and tiny screen. Welp. I'm going to use it on phone despite these difficulties but still it'd be nice if you could fix that.