HKEY_CURRENT_USER\Software\Classes\Applications\avidemux.exe\shell\open\command
is not erased/ set (at least when de-installing v.2.6 and then installing v.2.7)
Avidemux installer doesn't create this key, so it should not be removed upon uninstall.
So I must have created it manually (for v.2.6).
In Windows the key is useful because by it right clicking on a media offers "Open with" Avidemux.
Cause:
https://avidemux.org/smif/index.php/topic,18197.msg85608.html#new