Besides having over 500,000+ Chinese citizens and military personnel living in Russian abandoned territories.
China has now 'claimed' the Bolshoi Ussuriysky Island as entirely their own and Putin is too scared to do a thing about either of these.
Not only did the USSR bully China long ago and stole Chinese territory, but China sees the capitulation of the Communist USSR into 'capitalist' Russia as a betrayal to Communism.
Now Russia is between a rock (Europe) and a hard place (China). If Russia capitulates on Ukraine as a resource drain and goes from its former Pro-Asian expansionism to a Pro-European pacifist - you can bet your bottom dollar that China will take this as a threat and invade Russia officially.
It makes sense for China to take out its aggression against Russia - Siberia is rich in resources and gives the Yellow Man passage to the north once again.
This is the beginning of the end for the once mighty Russian (Soviet) entity throughout history.
In regards to the Soviet-USSR expansion east into Asia to the Berring Sea... with their default Cold War with America: General Macarthur said in the Korean War
"We are here (with the USSR) to kick out the yellow Mongol from Asia." with the master plan of an all-white northern hemisphere (Yin/Yang).
...but that master plan didn't work, did it?

Don't worry. While China is pounding the Russians and giving India some stick as well. Itchy-paranoid North Korea won't be able to help itself and it will bring the USA into the global conflict.
Told yas