Right now Designer's panel has two kinds of behavior:

1. click, move, and place
2. drag, and place

Some comments from wmedvede@redhat.com:

1) you click on the node, then you move for selecting the desired position on the canvas, then you click again for letting the node on the canvas
and
2) you click on the node, drag directly (i.e. don't let the mouse move up), select the desired position on the canvas, and finally "release" the mouse button (let the mouse button move up) for letting the node on the canvas.

This lead to two different issues:

1. double click on any node on the palette and new node will appear under palette
2. mouse down on any element on the panel, move mouse pointer for several pixels, release button - new node will appear under palette

More from wmedvede@redhat.com:

In my opinion it's OK to keep both 1) and 2). Of course the palette functioning can always be reviewed, we'll for sure find improvements to do. But this are not bugs.
If uses don't want to see the nodes "going behind" the palette, there is a "Process Designer" preference "Auto hide category panel" that can be set. When this preference is set to true then the palette is automatically hidden when we detect that user is doing 1) or 2). But yes, this is probably something that can be reviewed with UX, probably we want the "Auto hide category panel" to be set to true by default. Or even more palette functioning improvements.