Well, t he results are less conclusive when I look at the Service Test of the touchscreen which you can access by dialling *#*#7378423#*#* -> Service Test -> Touch screen In this case, the traces left on the screen by a finger are much thinner and thus - after magnification with a triple tap as an accessibility setting - provide a better picture of what happens at the edges. It seems that, sadly, some of the sensors might actually be failing very closely to the edges. This is impossible to see with the built-in diagnostics tool which I used before because it creates much thicker lines possibly colouring in areas where the sensors are actually failing. Maybe pulling down the bar on the lock screen works because the bar is already visible and hence covers areas with live sensors?
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Here is an observation indicating that this problem is likely to be a software issue: Doing the diagnostic test for the touchscreen, I get the inactive areas at the top and the bottom of the screen like everybody else. However, if I magnify the areas near the edges with a triple tap, I am able to completely cover them without fail. To do so, I actually need to wipe across the regions which - without magnification - were not responsive. This indicates to me that we are not facing a HW problem: the digitizer is working perfectly fine near at the top and bottom edges once the resolution has been changed (some people reported the change of the resolution as a workaround on rooted phones).
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