Middle-click is not currently supported by GoldenDict. The options available are in Edit > Preferences > Scan Popup.
If you use Windows, you might use AutoHotkey as a workaround as explained here
viewtopic.php?f=4&t=1431&p=7503&hilit=middle#p7503Hotkeys and ScanPopup are the only functionality in GoldenDict that are not cross-platform, i.e., they are implemented in a different way for each platform (Linux, Windows).
You can download the source code from
https://github.com/goldendict/goldendict and start looking at "goldendict\mouseover_win32" files.
Contributions are welcome
