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The overlay image comes from a reference in a configuration file that defines the “RoundCornerDisplay”. Multiple files are associated with this definition, all of them appearing to assist in avoiding drawing over parts of the screen to accommodate the unique display.
Referenced notch
Another file has the word "notch" in its name “ro.config.hw_notch_size” with the value set to “258,84,411,27”. It might represent left, top, right and bottom offsets in moving screen content.
With the new screen, Huawei might introduce a new 2240 x 1080 resolution. The device will have a 6.01” Samsung-made 2160x1080 pixel LCD, so it would be interesting to see how the manufacturer will implement the extra pixels on top.