I have an older program I use in groupy but it won't group the taskbar buttons into a single button (when the option is enabled). The way it works is the main window is an owned window. The owner is size to be 0x0 So it isnt visible but has the WS_VISIBLE bit set.
Groupy groups the owned windows like it should however it leaves all the taskbar buttons (which are the owners not owned). Sometimes while cycling through the tabs in groupy it will leave some visible. It is entirely possible to see 2 buttons for a single tab in groupy.
I can share some pics of it in action as well as some from spy++ if it would be helpful.
One thing I have noticed is in Windows 10 when I open the program If I move it to another monitor the taskbar button stays on the primary taskbar. I do have it set to "Only the monitor its on" in settings. However I use Actual Multiple Monitors which manages to move the TB button to the correct monitor.
I just noticed that when minimizing one of the grouped windows it does not minimize the group. Same for Close. If I close the the entire group from the button on the tab bar it works. Perhaps adding an option for min, max, restore as well as close?