Presently, it is impossible the know when your plugin window is brought to the front. There is no notification that reliably can give you this information, or from which it can be reliably deduced. I have reported on this substantially here:
In an app, you have TopLevelWindow::activeWindowStatusChanged(), and Component::broughtToFront() (for the window), and the windows themselves gain and lose focus - there are multiple ways to determine if a window has come to the front.
In a plugin, nothing.
One may wish to direct focus to a certain component, or reset certain parameters, or clear memory, or… whatever. Honestly, I don’t even believe that some sort of compelling use case is required to justify this request. It should just be in there - of course you would want to know when the plugin window is in front, somehow. It would have many possible applications. Thank you!