Detect pressed keys (Ctrl, Alt, Shift or Meta), code for https://addons.mozilla.org/addon/custom-buttons/ or something like http://userchromejs.mozdev.org/

detectPressedKeys.js

// Allow styles like
// :root[cb_pressed] { /* Pressed any modifier */ }
// :root[cb_pressed~="shift"] { /* Pressed Shift */ }
var pressed = {
	ctrl:  false,
	alt:   false,
	shift: false,
	meta:  false,
	__proto__: null
};
function detectPressed(e) {
	var type = e.type;
	if(type == "focus" || type == "blur") {
		// Just reset: not correct, but good for most cases
		for(var key in pressed)
			pressed[key] = false;
	}
	else {
		var isPressed = type == "keydown";
		switch(e.keyCode) {
			case e.DOM_VK_CONTROL: pressed.ctrl  = isPressed; break;
			case e.DOM_VK_ALT:     pressed.alt   = isPressed; break;
			case e.DOM_VK_SHIFT:   pressed.shift = isPressed; break;
			case e.DOM_VK_META:    pressed.meta  = isPressed; break;
			default: return;
		}
	}
	var state = [];
	for(var key in pressed)
		if(pressed[key])
			state.push(key);
	var stateStr = state.join(" ");
	if(stateStr)
		document.documentElement.setAttribute("cb_pressed", stateStr);
	else
		document.documentElement.removeAttribute("cb_pressed");
}
addEventListener("keydown", detectPressed, true);
addEventListener("keyup",   detectPressed, true);
addEventListener("focus",   detectPressed, true);
addEventListener("blur",    detectPressed, true);
addEventListener("unload", function destroy() {
	removeEventListener("unload", destroy, false);
	removeEventListener("keydown", detectPressed, true);
	removeEventListener("keyup",   detectPressed, true);
	removeEventListener("focus",   detectPressed, true);
	removeEventListener("blur",    detectPressed, true);
}, false);

examples.css

/* Show preview in Tab Scope only if Shift pressed */
#main-window:not([cb_pressed~="shift"]) #tabscope-popup stack {
	display: none !important;
}


/* Show preview in Tab Scope if pressed any modifier */
#main-window:not([cb_pressed]) #tabscope-popup stack {
	display: none !important;
}