Try it on Edge Browser, Edge has its own flash built in.
I have never seen this before. Make sure your setup to use two desktops also. Windows 10 has that capability.
On your taskbar there is a box, 3rd from the left, click on it and see how many screens you have.
When you play, you should only have the gameclient open. Nothing else.
Most of us have the gameclient bookmarked so that it opens in the browser window