The GPAC installer corrupts the PATH environment variable whenever it is long enough. In a few tests, the limit seems to be around 1024 characters: whenever the path variable plus the GPAC directory is larger than this, it is simply replaced by the GPAC directory.
Steps to reproduce:
1. Backup your PATH environment variable contents (optional but highly advised!);
2. Make sure the PATH environment variable contains more than 1024 - (GPAC directory string length);
3. Make a GPAC full installation;
GPAC would be installed and path added to PATH environment variable.
GPAC is installed; the PATH environment variable is corrupted.
* Skip "MP4Box" component during the installation. This will cause the installer to not touch the PATH environment variable.
* If "MP4Box" is needed, one may back up the PATH environment variable and manually set it afterward ("Control Panel", option "System", tab "Advanced", button "Environment Variables") -- note that GPAC directory should be added to the string.
I'm not sure if this is caused by some NSIS issue or by the GPAC installation script. I've taken a look at the script and at possible related NSIS bugs and found some relevant stuff although nothing in particular (sorry but I've lost the links).
My current suggestion goes to updating the NSIS version used and also review the scripts used (some appeared to be imported so the issue may be there).