Camera goes below ground and the screen turns black

• camera.positionCartographic.height reads -1840.5 after a tilt.
• The screen goes black or shows the far side of the globe.
• Dragging further makes the earth appear from the opposite side.

1. Read viewer.scene.screenSpaceCameraController.enableCollisionDetection in the console.
2. Listen to camera.changed and warn when height drops below 5 metres, printing the call stack.
3. Grep for flyTo and setView and inspect the destination height literals, since 0 or negative values land underground.
4. Check minimumZoomDistance, because a value of 0 lets the camera reach the surface.
5. Check whether terrain is enabled, since collision then tests against terrain height.

Re-enable enableCollisionDetection, set minimumZoomDistance to 30, minimumTrackBallHeight to 10 and a positive pitch limit, and always give flyTo a positive height such as Cesium.Cartesian3.fromDegrees(116.391, 39.907, 1200) with pitch -35 degrees.

var carto = viewer.camera.positionCartographic;
console.log(Cesium.Math.toDegrees(carto.longitude),
            Cesium.Math.toDegrees(carto.latitude),
            carto.height);
// 116.39 39.90 -1840.5

viewer.camera.changed.addEventListener(function () {
  var h = viewer.camera.positionCartographic.height;
  if (h < 5) {
    console.warn('相机高度过低', h);
  }
});
viewer.camera.percentageChanged = 0.01;

• Copying a free-camera example that sets enableCollisionDetection to false and never restoring it.
• Turning on depthTestAgainstTerrain and reading a black screen as an underground camera, when in fact clamped labels simply disappear.