Capturing input actions (CTRL+LEFT_DOWN)

CesiumJS
1

I would like to capture the center of rotation when using CTRL + LEFT_DOWN:

...

viewer.screenSpaceEventHandler.setInputAction(
  (payload)=>console.log(`CTRL-LEFT: ${JSON.stringify(payload)}`), 
  Cesium.ScreenSpaceEventType.LEFT_DOWN,
  Cesium.KeyboardEventModifier.CTRL
);

viewer.screenSpaceEventHandler.setInputAction(
  (payload)=>console.log(`CTRL-MOVE: ${JSON.stringify(payload)}`), 
  Cesium.ScreenSpaceEventType.MOUSE_MOVE,
  Cesium.KeyboardEventModifier.CTRL
);

The second input action (CTRL+MOUSE_MOVE) is captured successfully by my handler, but not the (CTRL+LEFT_DOWN) input action.

The behavior in the viewer is much as I’d expect (rotating the scene about a point), but why won’t my handler get invoked?

Thanks,