In a new project I select all the layer and delete it leaving a trensparent background (though RGB part is still hex: FFFFFF)
From there whether I do anything to the background layer or not, when I create more layer and then draw some stuff and rectangle select one object and drag it over another in the same layer the supposedly transparent pixels for the former object overwrite the corresponding pixels in the latter object.
I have tried multiple selection modes including the one I immediately though would fix this: "Add (union)". But unfortunately it didn't in any way.
Here's an image showing this problem of mine:
http://sv.tinypic.com/r/sv49kj/6
Edit:
I have found a workaround by adding a new layer on top in which I add the top object, then select it , drag over the object in the below layer, and finally merge the above layer down into the below one.
But, I'd still like to know why dragging a (rectangle) selected object over another object in the same layer does not blend the surrounding fully transparent pixels in the selection away, and instead they overwrite the below object completely. Why is that ?