Disable pitch in Cesium 1.6

ScreenSpaceCameraController.js

function rotate3D(controller, startPosition, movement, constrainedAxis, rotateOnlyVertical, rotateOnlyHorizontal) {
        rotateOnlyVertical = defaultValue(rotateOnlyVertical, false);
        rotateOnlyHorizontal = defaultValue(rotateOnlyHorizontal, false);

        var scene = controller._scene;
        var camera = scene.camera;
        var canvas = scene.canvas;

        var oldAxis = camera.constrainedAxis;
        if (defined(constrainedAxis)) {
            camera.constrainedAxis = constrainedAxis;
        }

        var rho = Cartesian3.magnitude(camera.position);
        var rotateRate = controller._rotateFactor * (rho - controller._rotateRateRangeAdjustment);

        if (rotateRate > controller._maximumRotateRate) {
            rotateRate = controller._maximumRotateRate;
        }

        if (rotateRate < controller._minimumRotateRate) {
            rotateRate = controller._minimumRotateRate;
        }

        var phiWindowRatio = (movement.startPosition.x - movement.endPosition.x) / canvas.clientWidth;
        var thetaWindowRatio = (movement.startPosition.y - movement.endPosition.y) / canvas.clientHeight;
        phiWindowRatio = Math.min(phiWindowRatio, controller.maximumMovementRatio);
        thetaWindowRatio = Math.min(thetaWindowRatio, controller.maximumMovementRatio);

        var deltaPhi = rotateRate * phiWindowRatio * Math.PI * 2.0;
        var deltaTheta = rotateRate * thetaWindowRatio * Math.PI;

        // Add this to rotate3D.
        // https://groups.google.com/forum/#!topic/cesium-dev/Pjrjw9asXns
        if(defined(constrainedAxis)) {
            // Disable pitching hopefully.
            deltaTheta = 0.0;
        }

        if (!rotateOnlyVertical) {
            camera.rotateRight(deltaPhi);
        }

        if (!rotateOnlyHorizontal) {
            camera.rotateUp(deltaTheta);
        }

        camera.constrainedAxis = oldAxis;
    }