Mouse wheel should zoom in and out and you can drag things around with your mouse (click and drag the white part to move the whole view around).
Or go to GeoGebra to view full sized.
I am working on adding a refraction slider.
This is an adaptation of Mick West's 'Obscuration by the horizon' in GeoGebra.
I tweaked it a bit and added DistanceCameraToHorizon, made the Obscured object conditional (it was showing a height obscured even when Horizon point was behind the target), and added in a factor for refraction. I would rather have the Tangent line be curved but applying the factor to Earth radius is EXACTLY the same thing (just in different units). Common refraction factors used in surveying are between 7% and 14%.
Here is a another calculator that will factor in refraction and give you the distance at which you can see an object of some height. But won't calculate the height hidden at some distance:
The metabunk calculator is also fairly nice in that it gives you the formula data in detail, but doesn't take into account refraction: