Prediction of the upper and left edge blocks when the edge blocks are in-frame prediction.

The only way to introduce the upper edge block is to use the above pixel (not present) (for example, vertical mode) and the left edge needs to use the left pixel (not present) (for example, horizontal mode).

Processing of the Edge block of H. I frame
The first left edge block of the I-frame, with intra-frame prediction, left and top without pixels, so at this point the left and top pixels are considered to be 128 (255/2), where the upper edge block makes a vertical prediction, because none of the upper edge pixels
All take a value of 128, while the left edge, because there is no left edge pixels, so in the horizontal prediction mode, the use of 128. Remember that the block predictions for I frames are based on the blocks that have been rebuilt on the left and top of the perimeter.
To be predicted again.