I have this same problem. The display freezes, but the phone responds. For example, pressing the power button twice causes the phone to vibrate as if the camera activated, or holding the power button for a few seconds and tapping where the restart option should be causes the phone to reboot (but removing the battery is usually the only option that works).
This problem always happens, in my case, when I try to record or listen to audios in WhatsApp. @rkbwde , in your photo I see this is the same case, isn’t it? It also happens, though rarely, when the alarm goes off. I disassembled, clean and reassembled the phone, and also did a factory reset, both to no avail.
@VashSan Yes, we quite often have speckles on the display when playing audios in WhatsApp. However, the phone doesn’t freeze always then.
“Unfortunately” (from a debugging perspective ), after swapping displays between our two phones and cleaning the pogo pins we haven’t experienced freezes recently. If it happens again, I will try pressing the power button and tapping blindly as you did (or pressing it for 15 sec to force a reboot) to see if the buttons still work when display is frozen.