I tried to C(opy) a symbol from a hierarchical sheet to its parent simply by dragging floating symbol up to the top icon area to "Leave sheet" icon. Clicking the icon we really left the hierarchical and got back to the parent, but instead of keep dragging the symbol, the whole hier.sheet dragged on (issue A).
In this state when I try to drop (the symbol or the hierarchical sheet, whichever) down, nothing happens clicking the left mouse button (issue B). Finally I push Esc to leave/move back hier.sheet to its original position.
Now I double click on the hier.sheet again. Instead of moving into the sheet, we stay at the parent, but hier.sheet jumps back to its last temporary position of when Esc has been pushed (Issue C).
If I drop the sheet to a new position, then try to Undo, it can't be un-done. (Issue D)
These are not serious issues, as I tried to do "something stupid", and that caused the weird behaviour, still...
That is indeed an interesting bug. We will have to disable changing sheets when copying objects. This would wreak havoc until we implement undo/redo across sheets.