1) Open a browser window on a 144 dpi monitor 2) Run `screencapture -x -i screenshot.png`, press the [space] key, and screenshot the window. 3) Run `sips -g dpiWidth screenshot.png`. The DPI is incorrectly set to 72.000, it should say 144.000. When these screenshots are used, e.g. in an email (MacOS Mail), they will be shown at twice their normal size. Firefox correctly sets the DPI value. Chrome also sets the DPI correctly when the window is opened - but does not update it when moving between monitors. As in, when you drag the window over to a 72 DPI monitor, the screenshots will still be recorded as being 144.000 ( https://crbug.com/793312 )
Please file this bug at bug report.apple.com. It's not a WebKit bug.
Thanks Simon, it has been raised as bug 35955716 https://bugreport.apple.com/web/?problemID=35955716