Ultra-thin 5K + 4K docking station problems on TB

Maybe not specifically for MacBook Pro, but since I have a Thunderbolt 3 dock in tandem with my LG Ultra Thin 5K display and LG Ultra HD 4K display, I would like to share my experiences.

I have a MacBook Pro M1 Max 10/32 14 “and I am using a legato Thunderbolt 3 Pro dock to which I have connected my 5K via Thunderbolt and the 4K display on the display port connector on the same docking station.

It works fine – but with some issues I initially thought they were Monterey bugs. The 5K screen will look like a 4096×2304 pixel screen instead of 5120×2880. I thought it was just weird scaling so I set it to 5K resolution in the scaling dialog. It works fine and in the system profiler everything looks good. CORN. This is not the case. * insert drama *.

So what seems to be happening is bandwidth is tight, reducing the data from the 5K display to “real” 4K or something (4096 pixels instead of 3840). Either way, the screen has a wobbly resolution and it’s not really that easy to detect at first because the screen has so many pixels and the difference in scale isn’t very, very noticeable. However – there it is.

It took me a while to figure out, but after some experimentation I now know that I need to run both displays from one port on the computer each. So I decided to run 5K directly on the laptop and 4K through the docking station. This also changes the 5K screen scaling dialogs so that there is a correct default resolution.

Now both screens look really crisp and good!

I’m including a screenshot of the system profiler data versus display data on my Mac to show how confusing things are. And to add to the confusion, the built-in screen, if used, will never show “user interface looks” only at screen resolution, so it’s unclear if the default mode is really correct. I had to measure the amount of pixels with the screenshot tool to see properly. I’m guessing this is because Apple is using its laptop screens in a default resolution that doesn’t match pixels between 2016-2020 pre m1 pro / max. A bit silly to have to hide this fact though. PS, macOS seems to be launching the names of the screens as well, but whatever. I’m already too confused!

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