The Qt documentation for QWidget::activateWindow() states:
On Windows, if you are calling this when the application is not currently the active one then it will not make it the active window. It will change the color of the taskbar entry to indicate that the window has changed in some way. This is because Microsoft does not allow an application to interrupt what the user is currently doing in another application.
However, Skype appears to defy this rule. If Skype is running but is not the active application, I can launch it from the start menu and it brings the existing instance to the foreground, activates it and grabs input focus.
And how can I do this?
(NOTE: This is specific to how QtSingleApplication works)
The solution is stupidly simple for my issue. Simply call AllowSetForegroundWindow(ASF_ANY); at the beginning of the application, and the original process will thus be allowed to bring itself to the foreground by use of SetForegroundWindow(). No strange hacks, just one line of code to add and no need to modify QtSingleApplication either.