Since I seem to be complaining about the S8 this morning, I discovered this today.
Previously when I connected it to various 1080p and 4k displays I just accepted that it was “as designed” that the image was cropped, to match the 16:10 aspect ratio of the built in display.
However there is a setting buried in the Dex settings, that is auto start Dex when connected to HDMI. And that works exactly as expected, but was a surprise was that it also then ran dex full screen with the proper 16:9 aspect ratio on 4k and 1080p displays.
It even (sort of) works on something like the LG Ultra Wide displays, filling the screen, though the UI and text are somewhat fuzzy, likely due to poor scaling on the display.
But why this isn’t part of dex starting regardless (perhaps a dialog box that asks if you want to run at the full supported resolution) is just beyond me.
Anyway, I’m glad to have found this, so enough griping for now.
PS: this also makes the option to use the internal display as a giant trackpad much more useful.