S4 Screen Dims after a while when on charge

[TheDufster]

Strange thing - I've just got my S4 back from repair and it's been reset to factory settings.

I've just finished restoring everything and setting things the way I like however there is one issue I can't solve.

I have set the Developer option to "Stay Awake" whilst charging. But after about a minute the screen dims. I noticed it today whilst in the car listening to BBC iPlayer Radio.

I know for a fact that it never used to do this and it used to stay on full brightness whilst I had it on charge in the cradle in the car.

I've checked through all the settings but I can't work out how to fix it.

Display > Brightness > Automatic Brightness = off
Display > Auto Adjust Screen Tone > off

 

[TheDufster]

Ok - I've just changed the setting Display > Screen Timeout to 10 minutes and now it stays bright (it was previously set to 1 minute)

But I guess it will dim again after 10 minutes. My point is, it never used to dim AT ALL when on charge and the maximum for this setting is 10 minutes.

???

 

[MozartMan]

My "Stay awake" setting in Developer options is disabled. And I remember it was enabled before. WTH?