Define View#isVisible() and use it to only handle key events if the focused view is not hidden.

src/ui/Key.js

@@ -70,7 +70,7 @@ var Key = this.Key = new function() {
 			key = keys[keyCode] || character.toLowerCase(),
 			handler = down ? 'onKeyDown' : 'onKeyUp',
 			view = View.focused,
-			scope = view && view._scope,
+			scope = view && view.isVisible() && view._scope,
 			tool = scope && scope.tool;
 		keyMap[key] = down;
 		if (tool && tool[handler]) {

src/ui/View.js

@@ -193,6 +193,17 @@ var View = this.View = Base.extend({
 		this._zoom = zoom;
 	},
 
+	/**
+	 * Checks whether the view is currently visible within the current browser
+	 * viewport.
+	 * 
+	 * @return {Boolean} Whether the view is visible.
+	 */
+	isVisible: function() {
+		// TODO: Take bounds into account if it's not the full canvas?
+		return DomElement.isVisible(this._canvas);
+	},
+
 	/**
 	 * @param {Point} point
 	 */