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You should upgrade or use an alternative browser. I have a Mac Mini M4 connected to a Dell monitor through a USB 3 to DisplayPort adapter, and I get 2560 x 1440 with acceptable quality.
I also have the same monitor connected to a Mac Pro5,1, which I occasionally access via Screen Sharing from the Mac Mini M4. Since both are connected via Ethernet, I'd say I can work perfectly on the Mac Pro from the Mac Mini, as the latter displays the full screen of the Mac Pro at 2560 x 1440.
However, when I access the Mac Mini M4 from the Mac Pro, even after turning on the Mac Mini and achieving a resolution of 2560 x 1440, I only get 1920 x 1080, with a black border around it, and the open windows rearrange themselves to fit the smaller size.
When I check the Mac Mini's settings via Screen Sharing, I see that the resolution is 1920 x 1080, and there's no trace of the connected monitor. However, when I switch the input on the connected monitor, the resolution remains at 2560 x 1440.
Am I doing something wrong, or does the Mac Mini M4 limit the pixel size through Screen Sharing? This is the first time I've encountered something like this.