In my case, I ran into this when plugging my N900 into my car and starting GPS-based navigation software. I was somewhat surprised when the display would not stay on because the battery was full. My point is, though indubitably technically accurate, how useful is this behaviour? What is the concievable benefit to the user of having the display switch off once the charging cycle is complete, instead of the implied "power-hungry display stays on when ample external power is available"?