The only problem on the iMac now is scrolling too often causing the window to reposition.
The window will reposition (instead of scrolling the content) if the mouse cursor gets too close to the right-edge of the window.
When the cursor is too close to the edge, you will notice the cursor shape changing from a regular arrow to a double-sided arrow (pointing left and right). That is the cue that a mouse-drag will reposition the window instead of scrolling.
If the cursor shape change is too hard to see, you can increase the cursor size from the Preferences > Accessibility > Display menu.
If you haven't, you may want to change the scroll bars setting in Preferences > General > Show Scroll Bars to "Always".
I'm also a fan of the Magic Mouse.