vPortYieldFromTick() returns to SIG_OUTPUT_COMPARE1A() where the final instruction is a return from interrupt (RETI). A RETI instruction assumes the next value on the stack is a return address placed onto the stack when the interrupt occurred.
When the RTOS tick interrupt started the AVR automatically placed the TaskA return address onto the stack - the address of the next instruction to execute in TaskA. The RTOS tick handler altered the stack pointer so it now points to the TaskB stack. Therefore the return address POP'ed from the stack by the RETI instruction is actually the address of the instruction TaskB was going to execute immediately before it was suspended.
The RTOS tick interrupt interrupted TaskA, but is returning to TaskB - the context switch is complete!
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