The pso.exe file that is provided with the Ragol installers has been modified to disable mouse support. This is a fullscreen executable provided with launchers to choose between fullscreen/windowed.
The main use case of this executable is that the dgVoodoo2 mod allows players to play PSO PC with increased resolution, AA and AF, along with the ability to alt-tab out. dgVoodoo2 only works in fullscreen for PSO, however. Historically, playing like this would mean that you still couldn't really tab out of the game, as the game would grab full control of the mouse. It will no longer do this.
It can be downloaded from the PC download page, or you may connect to the patch server to get the new exe along with the updated rare item and common drop files.