Canon News breaks down why QPAF should be coming in the future.
Right now with dual pixel AF (DPAF) sensors, you can focus reliably while the camera is in the horizontal position and your edge of contrast that you are locking on are vertical. If the edge is horizontal (or parallel to the camera orientation) then it has extreme difficulty in locking on. This is because all the pixels are arranged in one direction for dual pixel AF. What Canon needs is a quad pixel, where the pixel is split up, not once but twice, allowing for different phase different arrangements.
This patent application specifically deals with suppressing the deterioration that may happen with extremely accurate focus detection.