$myform.textinput.$isvalid is true when it should not

Question

The whole form should not be valid until stuff[] has at least one item added to it. When the user enters in a value into the text box and then clicks add it then adds the value to stuff[], only at this point when a value has been added to stuff[] should the submit button be enabled. However, as soon as the user types anything into the text box without clicking add, and thus nothing in stuff[], it makes the form valid and enables the submit button.

<form ng-app="myApp" ng-controller="myCtrl" name="myForm" ng-submit="submit()" novalidate>
    <div ng-repeat="things in stuff">
        <table><tr><td>{{things}}</td></tr></table>
    </div>

    <input type="text" name="app" ng-model="input" ng-required="!stuff[0]" />

    <button ng-disabled="!input" ng-click="add()">
        <span> add</span>
    </button>

    <input type="submit" value="Submit" ng-disabled="myForm.$invalid" />

    <script>
        angular.module('myApp', []).controller('myCtrl', function ($scope) {
            $scope.stuff = [];
            $scope.input = null;

            $scope.add = function () {
                var l = $scope.stuff.length;
                $scope.stuff[l] = $scope.input;              
                $scope.input = null; 
            };
            $scope.submit = function () {};            
        });
    </script>
</form>

Answer 1

[EDIT] To answer your question directly: !stuff[0] is TRUE when there is nothing in the array, and FALSE otherwise. When it is TRUE, then input is required, making the form 'initially' invalid. As soon as you type something into the input, then the requirement is now fulfilled, meaning that the form is valid, and you can now click the submit button. The condition actually has nothing to do with actually putting something into the array.

This is fixable by attaching a condition to stuff.length as proposed by my answer below. It won't make the form invalid (which you can easily do with this condition elsewhere) but it will at least disable the submit button. [/EDIT]

I don't understand why you have ng-required there, as you are wanting to disable the submit button, meaning the logic should be attached to the submit button, not the input text box.

I would do this instead:

<input type="text" name="app" ng-model="input"/>

<button ng-disabled="!input" ng-click="add()">
    <span> add</span>
</button>

<input type="submit" value="Submit" ng-disabled="stuff.length == 0" />

Which will disable the submit button if there is nothing in stuff.