Window loses focus when moving mouse cursor during loading

Since I updated my Mac from El Capitan to Sierra I've got an issue with focus on my Qt Application window.

When I launch my app, if I mouve the mouse cursor BEFORE the applications shows up, the applications hasn't focus (the 3 Mac top left buttons are grey, window title is grey). See attached file "focus.jpg".
I can use my app, enter some text using keyboard, then, I click on a button that opens a modal Qt file picker dialog which is totally stucked (because the app still hasn't focus).

To have focus, I must launch the app without moving the mouse cursor before the app window shows up (which is easy to do with the trackpad, but not with a physical mouse). Or, if the app shows up without focus, I can click on another app then go back to my app and then it gains focus.