System Information
Operating system: Ubuntu 16.04
Graphics card: GTX 680

Blender Version
Broken: 2.8, 7467049055a7

Short description of error
When I have enabled option emulate 3 button mouse, UI on all different places behaves weirdly. Sometimes I have to click twice to confirm selection and sometimes sigle click trigers click and drag action. This happends in 3D viewport, outliner, node editor etc. For example (I'm using right click select) if I have two object overlaping each other, and I use Alt + Right click to invoke select menu, where i can choose which object I want to select, to confirm selection I have to click twice. Doesn have to be double click (meaning two quick clicks). And this bahevior appears everywhere but with different behavior. In outliner when I want to change visibility for example, sometimes it clicks and grags ... meaning I am selecting visibility icons ion multiple objects. In node editor I want to change value of slider, after single click is invoked click and drag action.

Exact steps for others to reproduce the error
Most consistent way to reproduce this bavior is having active options Right click select and Emulate 3 button mouse. Then add two object overlaping each other (Monkey and Cube for example). Alt + Right click to invoke selection menu. To confirm selection it's needed to click twice. Without Emulate 3 butt.m. this behaves normaly.