Power went off while under high load.
I might have heard a snap noise coming from the laptop, but I cannot be sure, maybe it came from the speaker.
When reconnecting the power supply, charging LEDs come on on the laptop, as if charging the battery.
When trying to start the laptop, then either the LED light briefly turns green for 1/10th of a second, then nothing happens, or the blue LED blinks. In both cases t the power supply goes offline, i.e. the LED on the power supply goes off and won't come back on until de-plugged and re-plugged in a socket. This suggests an electrical problem within the laptop, maybe a short. The power supply also turned itself off when I accidently shorted it.
When holding down the power button of the laptop, a very faint clicking noise came from the power supply.
I also noticed USB ports were at least partially powered.
I checked the power supply with a multimeter and it delivers 19V.
There was not the slightest burn smell coming from the laptop.
=> so it might be a number of things.
1) the power suppply looks ok, but maybe it's shot somehow and despite testing ok for voltage is unable to deliver the required amperage of 11. something. but it's unlikely.
2) the long defunct battery interferes with the laptop's operation. shouldn't be the case as it should be able to operate without, but who knows.
3) the power jack/internal connection is faulty. unlikely because I almost never plug it in or out, the power jack doesn't move.
4) a protective fuse fried just behind the power jack: this might have happened. I was too agitated to think about a fuse. If that's the cause, it will be an expensive fuse. this scenario is compatible with the total absence of burn smell, but then I wonder why trying to turn on the laptop turns off the power supply.
5) something more nasty past the power jack assembly: I have no idea. I know there are one or two chips on the motherboard responsible for brokering the power to the mainboard's components, like the CPU, card slots, USB ports, etc. maybe it's one of those chips that failed. that's compatible with all symptoms.