LXDE-pi -- cannot show desktop on local monitor, but VNC is fine

Question

I'm setting my RaspberryPi 3B+ (Debian Buster) up as a magic mirror. I've run into a really weird issue that I can't wrap my brain around.

The magic mirror software is basically an EletronJS app, configured to run full-screen when LXDE-pi loads up. I have a monitor connected directly to the RPI, and have been configuring it via SSH.

I can set up the app to render various non-fullscreen components, and it renders properly on my monitor. Its appearance over VNC looks identical.

Adding any full-screen component causes my local monitor to display the new content momentarily -- before blanking to a black screen. Over VNC, the app displays properly (i.e., in fullscreen with the new content).

If I "restore" the ElectronJS app over VNC -- i.e., make it non-fullscreen and un-maximized -- then it suddenly appears on my local monitor.

If I resize the app window over VNC, I will see it resize on the local monitor -- up to a certain size, at which point the whole monitor goes black again.

---

/etc/xdg/lxsession/LXDE-pi/autostart:

@lxpanel --profile LXDE-pi
@pcmanfm --desktop --profile LXDE-pi
@xscreensaver -no-splash

@xset s noblank
@xset s off
@xset -dpms
# @unclutter -idle 0.1 -root

The app is re-/started/stopped using PM2, which in turn delegates to a shell-script -- ~/mm.sh:

#!/bin/bash
cd ~/MagicMirror
DISPLAY=:0 npm start

Finally -- my HDMI resolution is set to be DMT Mode 47 (1440x900, 60Hz) -- /boot/config.txt:

...

hdmi_group=2
hdmi_mode=47

# We ran into trouble using the experimental GL driver; so we're using the legacy, non-GL version
# dtoverlay=vc4-fkms-v3d

gpu_mem=256

# Finally -- the display is rotated 90 degrees CW to a portrait orientation
display_rotate=1
avoid_warnings=1
...

Answer 1

Well, I don't know why this would be the answer, but it's working now.

I had to radically moderate my expectations w/r/t display resolution.

It seems that, so long as I'm displaying nothing complicated -- e.g., white text on a black background -- I can use higher resolutions (like that 1440x900@60Hz I was using before).

But the moment I try to display any image that fills more than so-much of the screen, the screen blanks completely. (This appears to be related to the HDMI controller rather than X, because my VNC sessions are unaffected.)

The only way I can achieve image-display is by adopting a lower resolution; in my case, by going with CEA mode 4 (1280x720@60Hz).

Note that this is after bumping my allocated graphics RAM to the maximum (256MB), and disabling the experimental GL driver.

My /boot/config.txt ends up looking like:

#hdmi_safe=
#disable_overscan=
#overscan_left=
#overscan_right=
#overscan_top=
#overscan_bottom=
#framebuffer_width=
#framebuffer_height=

hdmi_force_hotplug=1
hdmi_group=1
hdmi_mode=4
hdmi_drive=2
config_hdmi_boost=6

# ...

[pi4]
max_framebuffers=2

[all]
#dtoverlay=vc4-fkms-v3d
gpu_mem=256

display_rotate=1