This patch introduces a user preference setting for reducing the Eevee viewport resolution.
This feature has been requested in this Right-Click Select submission (which I found after I started writing the patch). It's highly desirable for GPUs which are not powerful enough to keep up with the resolution of the display. For instance, this applies to many of the mainstream Macintosh models with a Retina Display (including the one I have) and without it editing in the LookDev/Rendered mode can be very difficult.
This video demonstrates the feature in action. Note that the video is based on an older version of the patch and has glitch adjusting the rendering scale. The version posted here doesn't have the glitch.