set otherplayer as master client when master client leaves room photon

Host migration (which is the buzzword here) is a quite complex topic. As you can read in the photon docs:

There is no generic solution in Photon for "Host Migration". However, we make things much easier for you.

Also:

Photon does not hand all information that the former Master Client had about the room's state to the newly elected one.

So you have to handle most things yourself. What exactly soley depends on your own architecture. There are two main scenarios that need to be distinguished:

• The host suddenly lost the internet connection.

• The host is actively promoting one of the other clients to become the new master.

The first case is much more difficult because in order for a client to take over as master, every client has to have a copy of the whole gamestate. This is generally actually not a good approach for most games since it's usually the master / host that has certain secret information that would give cheaters an advantage. The host / master could always cheat. However if the whole gamestate is always shared with all other clients, everybody could see any information there is about the game.

The second case is a lot easier since the current master can actively send the relevant data to just one client. This can be done either on command or when the host actively clicks a disconnect / leave button. Of course such a migration would not be possible if the game is force-closed. For example for WebGL builds force closing is the most likely case as people would simply close the browser tab.

As I already said, noone can tell you what information the current master should or have to send to the newly choosen master. That's up to your game.