There are complicated relationships in the tech industry. And then there is the rocky marriage between Apple and Samsung Electronics.

While Apple has long depended on the Korean electronics giant to supply key parts for the iPhone, it has also waged a long-running court battle against Samsung, accusing it of ripping off Apple patents in its own line of smartphones. The two companies settled their legal squabbles five years ago, and Apple found other partners to supply some ingredients in its devices like memory chips or replaced other Samsung components such as processors with its own designs.

Still, a complete breakup wasn’t an option, because Apple still needed Samsung for one vital component: displays for iPhones and other devices. The result is an unusually tense working relationship.

Samsung so distrusts Apple—which is on a quest to find alternatives to Samsung’s displays—that it bars Apple engineers from Samsung factories, according to multiple former Apple employees. In one incident in 2017, Apple engineers flew to South Korea from the U.S. to meet with employees at Samsung’s display division but were told they weren’t welcome inside its facilities, including its office buildings, because Samsung had to protect its intellectual property around a screen technology known as organic light-emitting diodes, or OLEDs. Instead, Apple’s engineers were forced to speak with their Samsung counterparts remotely from their hotel rooms in South Korea, according to a former Apple employee briefed on the matter.