I'm running emacs 26.1 on ubuntu 18.04 on a Dell laptop with a high-resolution display. It works very well in that configuration. It also works very well when the laptop is plugged into a lower-resolution monitor.
What doesn't work is that when you attach or unplug the monitor, emacs does not adjust to the change in resolution. So when emacs was started without the monitor attached, and then later the monitor gets attached, text is very large in its physical dimensions. Going the other way, unplugging the monitor yields unreadably tiny text.
Other applications (e.g. firefox and gnome-terminal) don't suffer from this issue, presumably because they react to some signal from X or gnome and adjust to the change in resolution.
Is there any workaround for this problem other than restarting emacs? If not, what work needs to be done to emacs to make this work better (bug references?)?