Infinite scroll in Palette of diagrams (GMF and Sirius)

An infinite scroll, to top then to down, appears on some circumstances.

Steps to reproduce

• Create a new class diagram (or open an existing one)
• Scroll the "Nodes" sections to the bottom (two clicks for example)
• Collapse the Edges section
• Reopen it
• Scroll the "Nodes" sections to the top (one click for example)
• An infinite scroll "top then down" appears (like in this video : scrollProblem.mkv)