Great info. I had read through the spec and found that, but doesn't that apply to port (source) design? From what I understand, a USB DCP (dedicated charging port) has the D+/- lines shorted, allowing the portable device to identify it as such.
But I'm designing the portable device, not the port. Do the same rules still apply? I.e. should I short the D+/- lines on my design?
You are correct that the linked spec applies to the charger, not the device being charged/powered. You are incorrect to suggest that you should be shorting the D+/D- lines on your device. Obviously, you should be
testing whether you're are plugged into a charger with shorted D+/D- lines, and only turning on your display iff you are plugged into a charger with shorted D+/D- lines. You're receiving the "I can support 1.5A" message, not sending it.
This means that you won't be able to draw 1.5A from computer USB ports, because of course computer USB ports don't have their D-/D+ lines shorted. There
might be a way to communicate to the PC over the D+/D- lines to request more power; I don't know if the USB spec allows for this. From my
out-of-date knowledge from years ago, that requesting mechanism only goes up to 500mA, you cannot "legally" draw more than 500mA from a computer USB port. And indeed, modern phones plugged into modern computers with a micro-USB cable only pull in 500mA.