As discussed over at www.stackoverflow.com/questions/14893988, binding the functionality of a keystroke to an action instead of a state will establish noninterrupting events while still preserving individual keystrokes.

kbdin.js

var kbdin = 
{
	downbinded: new Object(),
	upbinded: new Object(),
	stroked: new Object(),
	
	downbindKeystroke: function(keyCode, functionality)
	{
		this.downbinded[keyCode] = functionality;
	},
	
	upbindKeystroke: function(keyCode, functionality)
	{
		this.upbinded[keyCode] = functionality;
	},
	
	isDownbinded: function(keyCode)
	{
		return this.downbinded[keyCode];
	},
	
	isUpbinded: function(keyCode)
	{
		return this.upbinded[keyCode];
	},
	
	isStroked: function(keyCode)
	{
		return this.stroked[keyCode];
	},
	
	onDownstroke: function(event)
	{
		var keyCode = event.keyCode;
		
		if(!this.isStroked(keyCode))
		{
			this.stroked[keyCode] = 1;
			
			if(this.isDownbinded(keyCode))
			{
				this.downbinded[keyCode]();
			}
		}
		
		if(this.isDownbinded(keyCode))
		{
			event.preventDefault();
		}
	},
	
	onUpstroke: function(event)
	{
		var keyCode = event.keyCode;
		
		delete this.stroked[keyCode];
		
		if(this.isUpbinded(keyCode))
		{
			this.upbinded[keyCode]();
			event.preventDefault();
		}
	},
	
	handleKeystrokes: function()
	{
		for(var keyCode in this.downbinded)
		{
			if(this.isStroked(keyCode) > this.keyDelay)
			{
				this.downbinded[keyCode]();
			}
		}
		
		for(var keyCode in this.stroked)
		{
			this.stroked[keyCode]++;
		}
	},
    
	keyDelay: 5,
	keyFrequency: 50
}

window.addEventListener("keydown", function(event) {kbdin.onDownstroke(event);}, false);
window.addEventListener("keyup", function(event) {kbdin.onUpstroke(event);}, false);
window.setInterval(function() {kbdin.handleKeystrokes();}, kbdin.keyFrequency);