After click scroll bar.Mouse click trigger the keyboard event

I use Ctrl Event to make the function of continuous selection.But the issue occured. I clicked UMG’s scroll bar, then pressed Ctrl button and click for selection, then mouse click trigger the Ctrl released event at the same time.
This cause the continuous selection failure. How can I fix it?

http://i2.muimg.com/567571/3731611f4f573246.png

http://i2.muimg.com/567571/fc8702c510290964.png