U2720Q, disable USB Type-C as a source

@miric  Keeping a battery topped up at 100% isn't necessarily ideal for battery longevity.  That said, an idle battery wouldn't lose 12% of its charge overnight either.  Self-discharge takes quite a bit longer than that.  So it's possible that the MacBook itself is regulating its power draw.  The fact that it's constantly connected to power doesn't mean it will always draw power.  Apple has implemented "Optimized Battery Charging" on iPhones, iPads, AirPods, and the Apple Watch already, which initially only charges the battery up to 80% overnight and then only charges the final 20% shortly before it predicts you're going to start using it in the morning.  So maybe the MacBook is doing something similar.

Anyway, are you using HDMI separately because you have a MacBook Pro model that can't run 4K 60 Hz and USB 3.x over USB-C simultaneously?  You didn't specify the exact model you have (always a good idea, fyi).  But if that's the case, yes I agree that Dell dropping the USB-B port was an unfortunate move.  You can force USB 3.x data by setting the USB-C Prioritization option on the display to High Data Speed.  That works because your video signal from a DP 1.2/HBR2 system will now be limited to 4K 30 Hz rather than 4K 60 Hz to make more bandwidth available for USB 3.x, but it won't disable video entirely.  The only way I can think to disable video over USB-C completely would be to use a USB-C to USB-A 3.0 cable, and then a female USB-A to male USB-C dongle to connect to your MacBook Pro.  Of course it's a bit ridiculous to have to do that, and you'll lose charging that way too, but that's the only idea I've got.  It seems that Dell's 4K USB-C displays didn't plan for systems that ONLY had USB-C ports and couldn't run 4K 60 Hz and USB 3.x over USB-C simultaneously.

> Keeping a battery topped up at 100% isn't necessarily ideal for battery longevity

This is exactly one of my concerns.

> That said, an idle battery wouldn't lose 12% of its charge overnight either.  Self-discharge takes quite a bit longer than that.

However, it does. Normal usual behavior for my MBP with a closed lid, without any tune tricks to reduce power consumption in the sleep mode. My typical scenario with previous U2718Q was MBP on battery in the morning until it discharges to ~20% than the full day on an external wall adapter than turned off for the night and discharge to 88%. Repeat. I had an entire top case exchange this summer and now I try to stick with this cycle to keep the battery healthy.

> You didn't specify the exact model you have (always a good idea, fyi)

My fault, you're right.

I used to have the red corners issue with my secondary replaced U2718Q and was very glad I got a brand new U2720Q as a warranty exchange. Frankly, I hung up on the moment how to connect to U2720Q when I got it. The reasons are the absence of the USB-B upstream port, and the fact my MBP 15 2017 does not support the HBR3, and constant charging through the USB-C port. I know it's a trend now on the market and if Apple and other manufacturers push this type of connection, it must be optimal for the MBP battery though I'm in doubt.

Of course, I preferred the High Resolution setting opposite USB 3.x in this case until further computer upgrade. And on Catalina, my MBP handles 4K@60Hz,  30-bit only with a closed lid. Funny thing with just installed Big Sur it works with the same specs even with an open lid!

So my supposed connect scenario is (similar to the previous U2718Q):
— USB-C to MBP + USB 3.x priority setting to get fast monitor hub, and without charging (impossible as I see)
— HDMI 2.0 to USB-C cable to deliver video signal from MBP
— external wall brick to manually control MBP charge.

I've tried to cheat with the native MBP's white USB-C cable to drive the monitor's hub without video but it did not work. I have slow USB devices connected to the monitor (external Logitech webcam and headset). The reason why I'd like to have USB 3.x is an external HDD. And I see the only solution in my case is to drop the monitor's hub functionality at all for a while, and use my external D-Link USB 3.x hub with its wall brick to power the external HDD plus those slow devices. That brings me a wire mess. Otherwise, I should stick with the current connection scheme with a closed lid, a slow monitor's hub, an external HDD drive connected directly to the MBP, and to lose the charge control feature.