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Right, so here's the challenge, have British Tibet retain its borders (as seen in the below map) and have it survive into the present day.
Now, as for the What-If, well what would happen with a huge Tibet surviving, would Xinjiang (now connected to China by a relatively thin area of land) remain Chinese or go independent, will China fragment or Unify quickly, basically what will the consequences be?
NOTE:
The Tibet border is overlayed onto the OTL modern world to show where it is, it's not meant to show a lack of butterflies.