White hole on the screen

Hello guys,

Yesterday on my father's old iPhone 4 something strange happend. He was reading some news in Safari and his screen became white, but not the whole screen. He said that he tried to press HOME button, but nothing happened. Then he pushed the POWER button to lock an iPhone and there was lock sound (screen off). He pushed POWER button again, but the white hole was still there. After that he pushed HOME and POWER button to reset it and it was all just fine.

I am just interested what can be an issue and is that going to happen again?

Gray around, white inside and some darker gray line on the right side.