I performed some experiments, using NEC P241W monitor (resolution 1920x1200, DVI input), LG 22EA53 (resolution 1920x1080, DVI and HDMI inputs) and TV (resolution 1920x1080, HDMI input). As video sources I used MNT reform (set to display mode dual) and Purism Librem 13 (also MacBookPro from 2015 and small computer, but results were the same as for Librem 13, so I’ll focus on it and Reform).
Regardless of external monitor used, when I started Reform with HDMI cable in, image was only displayed on external monitor (nothing on built-in screen). Then, when I run sway, image was displayed on both screens, but internal one was unstable - it was oscillating between workspace 1 and 2, with frequency of about 1-2Hz. It made working impossible. On the other hand video displayed on external monitor was stable and usable. This situation occured with all external displays.
When I exited sway, again image on internal display disappeared, and was only displayed on external display.
When I started Reform without HDMI cable connected, and connected it after logging in (still in text/CLI mode), external monitor was not displaying anything. Image appeared after running sway - but with the same problems with blinking on internal screen.
But when I first started sway, and only then connected HDMI cable - everything was OK. Image was usable on both screens, and system was stable. I was able to run and use applications on both screens.
This was independent on external screen (NEC, LG, or TV) and on connection on its side (HDMI or DVI, so cable was either HDMI-HDMI or HDMI-DVI). Additional information - I have connected NVMe disk, but system was started and used SD card; during those tests NVMe was not accessed.
It’s not big problem for me - and I’m not sure if this can be solved with blob managing HDMI output. But I wanted to put this information, to see if others noticed similar behavior.
Post is getting long - so I’ll write about differences in available resolutions later.