XQuartz 2.7.11 on macOS Sierra 10.12.6, using 27-inch Apple LED Cinema display, with resolution set to 2048x1152.
using a browser I visit https://www.sven.de/dpi/ and enter diagonal=27 to get reasonable physical dimensions of the screen:
23.53" x 13.24" = 311.5in^2 (59.77cm x 33.62cm = 2009.68cm^2) at 87.03 PPI, 0.2919mm dot pitch, 7574 PPI^2
But using xdpyinfo (from macports) it reports:
dimensions: 2048x1152 pixels (541x303 millimeters)
resolution: 96x97 dots per inch
The pixels are correct, but the millimeters are too small: so DPI is too high.
The millimeters should be: 598x336.
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