1) Start SB
2) Drop an AnchorPane
3) Drop a Button in the AnchorPane and *keep the mouse immobile after the release*
4) Press and maintain the mouse button (over the Button component just dropped)
    => cursor changes from default to "closed hand" : BUG (should stay as default)
    => focus is transferred to Library panel : BUG (should stay in content panel)
5) Drag the button
    => the button does not move : BUG (it should move)

I put a break point in mousePressed handler of Content panel.
During step #4, this handler is not invoked : mouse pressed event
seems to be caught by someone else (the library panel ?).

I did the same test with Scene Builder Probe : it works fine.
So the root cause seems to be in the App.

This issue does not break anything.
However it's pretty annoying and smelly.