It definitely requires a much faster double-click than my system setting. I use double-click on empty taskbar space to open Task Manager and it responds to my usual double-click speed. However, what I have since discovered is that it's doing both the single and double click events. So if I have ctrl+lclick set to close all items, and ctrl+ldblclick set open a new instance, it will do the lclick event first immediately followed by the ldblclick event. I hope that makes sense.

"if the program is running as administrator"

Thanks, yes I checked and that definitely is the case with Notepad++. I'm not sure what I was doing before with IrfanView but it works fine. Perhaps an update could be made to the help file to specify the issue with administrator mode.

"Perhaps it happens with specific program(s)?"

It would appear so. I tried with Notepad as you had done and it worked. I also tried FirefoxPortable and that worked but IrfanView and Notepad++ didn't.

Ok my bad, I just realised it didn't work because I had set Grouping to Don't group.

But I did discover that Maximise all group items doesn't work. When I click an item nothing happens.