Password field on external HDMI monitor is only rendered after mouse movement

System

Linux Mint 22.3 Cinnamon (Zena)
Kernel: 6.8.0-137-generic
slick-greeter: 2.2.6+zena
LightDM: 1.30.0
GPU: Intel Iris Xe Graphics (Tiger Lake), i915
External display: Lenovo P27h-20, 2560x1440 via HDMI
Internal display: eDP-1, 1920x1080
X11 session

Problem

When booting with an external HDMI monitor connected, Slick Greeter displays the background, but the password input field is not visible.

The password field appears immediately as soon as the mouse is moved.

Keyboard input alone does not make the password field appear.

The problem is reproducible on every boot with the external HDMI monitor.

When booting with only the internal laptop display, the password field appears immediately and no mouse movement is required.

Steps to reproduce

Connect external HDMI monitor.
Boot Linux Mint normally.
Wait for Slick Greeter to appear.
Do not move the mouse.
The background is visible, but the password field is not.
Pressing a keyboard key does not make it appear.
Move the mouse slightly.
The password field appears immediately.

Expected behavior

The password field should be visible immediately when Slick Greeter is displayed, without requiring pointer movement.

Actual behavior

The password field is apparently already created but is not rendered/displayed on the external monitor until a pointer-motion event occurs.

Relevant Slick Greeter log

The log shows that the password prompt is created approximately 0.47 seconds after startup:

[+0.38s] DEBUG: main-window.vala:259: Monitor 0 is 1920x1080 pixels at 0,0
[+0.38s] DEBUG: main-window.vala:259: Monitor 1 is 2560x1440 pixels at 1920,0
[+0.38s] DEBUG: main-window.vala:275: MainWindow is 0x0 pixels
[+0.38s] DEBUG: slick-greeter.vala:832: Showing greeter
[+0.38s] DEBUG: slick-greeter.vala:314: Showing main window
...
[+0.44s] DEBUG: Starting authentication for user torsten...
...
[+0.47s] DEBUG: Prompt user with 1 message(s)

Despite this, the password field is not visible until the mouse is moved.

Additional testing

Internal display only: works correctly; password field appears immediately.
External HDMI monitor connected: issue reproducible.
Keyboard activity does not trigger the missing redraw.
Mouse movement immediately triggers it.
xrefresh does not make the password field appear.
A synthetic pointer-motion event using xdotool does make the password field appear.

For example, this workaround works:

xdotool mousemove_relative -- 1 0
xdotool mousemove_relative -- -1 0

Therefore this appears to be specifically related to a pointer-motion-triggered redraw/update of Slick Greeter in a multi-monitor configuration.

Additional graphics information

In the logged-in Cinnamon session:

eDP-1 connected
HDMI-1 connected primary 2560x1440+0+0

The internal eDP display is not active in the Cinnamon desktop session.

Please let me know if additional LightDM, Xorg or i915 logs would be useful.

Start by reproducing the issue with the external HDMI monitor, then trace the showing and prompt paths mentioned in slick-greeter.vala and the monitor setup in main-window.vala. Done means the password field is visible immediately on the external display without mouse movement, while keyboard-only input continues to work.