Changing primary display fails when HiDPI mode is enabled

[jacobgkau]

IMPORTANT
Please attach output of the xrandr command and ~.config/montors.xml to help diagnose the issue.
xrandr-1.txt
monitors-1.txt

Distribution - (run cat /etc/os-release): Pop!_OS 20.10

Related Application and/or Package Version - (run apt policy $PACKAGE NAME): 18.04.6~1594738317~20.10~cfe8850

Issue/Bug Description: On addw2, if I plug in an external display (ViewSonic VP2771), ensure HiDPI mode is enabled, and attempt to set the external display as primary, I end up with the external display zoomed in too far and no confirmation dialog visible.

Starting configuration:

• 1x 4k internal display (primary)
• 1x 1440p external display (secondary)

After attempting to change the configuration (the second display is scaled up larger than it was before, even though I didn't touch the scaling settings, only the primary display selection):

A screenshot shows that a confirmation dialog should be visible, but it's not on the screen physically:

After the dialog countdown finishes and the change "reverts," I'm left with the second display still scaled up too high (but no longer cut off):

Attempting to restart GNOME Shell to recover from this results in the internal display not being scaled up like it should, even though "Scale" is still set to 200% (for both displays):

Logging out and back in gets me back to where I started (good resolutions with the internal display as primary.)

Steps to reproduce (if you know): See above.

Expected behavior:

• The second display's scaling settings should not change because it's becoming the primary display.
• The screen should not be cut off and the confirmation dialog should be visible.
• After restarting GNOME Shell, the internal display should respect the currently-selected scaling settings.

Other Notes: If I set the daemon to LoDPI mode, I can set the external display as primary:

If I then try to turn the daemon to HiDPI mode, then I get a dialog that I either need to stay in LoDPI mode or switch the primary display to internal:

If this is an accepted limitation, then I should get a similar dialog when I try to switch the primary display while I'm in HiDPI mode, offering to either keep the internal display as primary or switch to LoDPI mode to achieve the requested configuration.