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MacOS detecting wrong screen size

My Macbook Pro is detecting two external monitors as completely the wrong size - in one case insanely wrong (122.5" !!!), and I can't even read the screen. Both the BenQ and Philips monitors are 27" 4k monitors. The Philips is connected through a USB hub via HDMI and the BenQ connected directly through a USB-C display port cable.


They used to work just fine, and at times random swapping of the cables between the ports used to fix things. How does one change or override the auto-detected screen size? I have not changed any settings on these monitors and they worked find at one point in time, now it is impossible to get them to work without scaling down the resolution. In the case of the BenQ, using the option key on the "scaled" options in displays doesn't even list the proper native resolution (3840 x 2160) of the monitors.


I had another USB hub (Geddie) that seemed to be the source of a similar problem when connecting over HDMI, so I swapped it for an AV FlePow hub and got things to work. Now, after upgrading to Big Sur the problem has returned with the new hub and no amount of fiddling and swapping cables has yet to correct it - this has to be a USB system issue of some sort. It is almost like there is a cached display resolution per thunderbolt port or device ID and once established or corrupted, no normally available system setting can change it without swapping things out to redo detection with a new USB device.


MacBook Pro 15″, macOS 10.12

Posted on Dec 1, 2020 6:44 PM

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Posted on Dec 4, 2020 11:48 AM

Oh, I guess because of the rotation, the correct resolution is flipped - 2160x3840 instead of 3840x2160. But still, the scaling of the text and mis-detection of the size is completely insane. After randomly flipping rotation, scaling settings and plugging / unplugging ports I was able to get my BenQ and LG monitors to work just fine. But I still can't stop the shrinking down that happens on the Philips, my "61-inch" 27-inch monitor - which should be obvious from the model number.


Again, ladies and djents, you have some bugs here to address. Hopefully this has generated at least an internal bug report. I am quite capable of providing any and all information required. My suspicion now is that connecting over the HDMI interface from a USB hub causes a mis-detection of an external monitor as a television and alters the size. In all honesty, it could be the USB hub HDMI interface that is the problem, and that a set of pre-existing bugs allowed me to get MacOS to cache the appropriate 27" screen size on the port to which the hub was connected, negating the USB / HDMI proble, which is why things worked before. But even if this hunch is correct, it in no way explains the bug whereby my directly connected display port cable allows for my Ben-Q to be detected as a 122.5" monitor.

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Question marked as Best reply

Dec 4, 2020 11:48 AM in response to zamsden

Oh, I guess because of the rotation, the correct resolution is flipped - 2160x3840 instead of 3840x2160. But still, the scaling of the text and mis-detection of the size is completely insane. After randomly flipping rotation, scaling settings and plugging / unplugging ports I was able to get my BenQ and LG monitors to work just fine. But I still can't stop the shrinking down that happens on the Philips, my "61-inch" 27-inch monitor - which should be obvious from the model number.


Again, ladies and djents, you have some bugs here to address. Hopefully this has generated at least an internal bug report. I am quite capable of providing any and all information required. My suspicion now is that connecting over the HDMI interface from a USB hub causes a mis-detection of an external monitor as a television and alters the size. In all honesty, it could be the USB hub HDMI interface that is the problem, and that a set of pre-existing bugs allowed me to get MacOS to cache the appropriate 27" screen size on the port to which the hub was connected, negating the USB / HDMI proble, which is why things worked before. But even if this hunch is correct, it in no way explains the bug whereby my directly connected display port cable allows for my Ben-Q to be detected as a 122.5" monitor.

MacOS detecting wrong screen size

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