The Bunker Group
Verified Defense Pro
Pakistan & China disagreed about the border until 1962, when China agreed to discuss the dispute - after Pakistan voted in favour of th PRC getting China's UN seat. They signed an agreement in 1963 which divided the disputed area, China getting most of it. Some people in Pakistan call it a cession by China, saying Pakistan (& before that the rulers of Kashmir) had never disputed China's sovereignty over the land it agreed was Chinese, & it was just a question of delimiting the border. Pakistan has had Chinese support since then.
The border in question is between Kashmir (Gilgit & Baltistan) & Sinkiang.