In Aframe 0.3.0 the mouse events do not fire correctly when multiple objects intersect the raycaster/cursor. Unfortunately, this makes the cursor events unusable for any UI that has overlapping objects. This worked as expected in 0.2.0.

Demo of the bug in 0.3.0: https://codepen.io/ryanbetts/pen/yaLVGL
Demo of it working in 0.2.0: https://codepen.io/ryanbetts/pen/bwGgoG