Why an external monitor changes emac's frame text size?

Question

I recently migrated to emacs 27 up from 24 (new machine running on Ubuntu 20.04, up from 16.04).

When connecting (or unplugging) the laptop to an external monitor, emacs fails to adapt text size to the new monitor's resolution. Moving to the external monitor with emacs open leaves text too big, too small when removing it with emacs open. But opening emacs after the monitor change keeps text size correct.

While this suggests workarounds, it involves a bunch of code. My previous machine need no tweak to .emacs in order to achieve monitor changes without affecting text size. I suspect the new laptop's superior screen resolution.

Does anyone see what may be going on?

Answer 1

I think you nailed the cause. Your new laptop has a higher resolution than your external monitor. If your resolutions matched previously (1080p for example), the text would appear to adjust to the new monitor, but was actually just reflecting the monitor's lower dot-per-inch (DPI). With a higher resolution on the laptop, the text resolution remains the same, but the DPI differences cause the text to appear as it does.