x11vnc -clip shows black box on shared screen

[christf]

Description I am using three screens (but the issue is also visible when using two). One of those is virtual. I running debian 10 stable. When sharing the virtual screen with x11vnc with the -clip option there is an area that is not drawn on and displays a black box.

To Reproduce

set up screens like this:

• HDMI1 on top
• eDP1 below HDMI1
• VIRTUAL1 right of eDP1

as outlined by the xrandr output below.

Then share the virtual screen with x11vnc using the clip option.

use the x11vnc -clip option like this:

x11vnc -display :0  -clip 2048x1536+1920+1024 -ncache 10 

This shows the virtual screen on the client (VNC Viewer on Ipad) however there is a black box overlaying parts of the screen

Expected Behavior

no black box on the virtual screen when using the clip option

Desktop (please complete the following information):

• OS and version: debian 10
• Xorg version used: xorg/stable,now 1:7.7+19 amd64 [installiert]
• Wayland version used: -

Additional context

xrandr 
Screen 0: minimum 8 x 8, current 3968 x 2560, maximum 32767 x 32767
eDP1 connected primary 1920x1080+0+1024 (normal left inverted right x axis y axis) 290mm x 170mm
   3200x1800     60.00 +  48.01  
   2880x1620     59.97  
   2560x1600     59.99    59.97  
   2560x1440     59.96    60.00    59.95  
   2048x1536     60.00  
   1920x1440     60.00  
   1856x1392     60.01  
   1792x1344     60.01  
   2048x1152     60.00    59.90    59.91  
   1920x1200     59.88    59.95  
   1920x1080     59.96*   60.00    59.93  
   1600x1200     60.00  
   1680x1050     59.95    59.88  
   1600x1024     60.17  
   1400x1050     59.98  
   1600x900      60.00    59.95    59.82  
   1280x1024     60.02  
   1440x900      59.89  
   1400x900      59.96    59.88  
   1280x960      60.00  
   1368x768      60.00    59.88    59.85  
   1360x768      59.80    59.96  
   1280x800      59.81    59.91  
   1152x864      60.00  
   1280x720      59.86    60.00    59.74  
   1024x768      60.00  
   1024x576      60.00    59.90    59.82  
   960x540       60.00    59.63    59.82  
   800x600       60.32    56.25  
   864x486       60.00    59.92    59.57  
   640x480       59.94  
   720x405       59.51    60.00    58.99  
   640x360       59.84    59.32    60.00  
HDMI1 connected 1280x1024+0+0 (normal left inverted right x axis y axis) 380mm x 300mm
   1280x1024     60.02*+  75.02  
   1280x960      60.00  
   1152x864      75.00  
   1024x768      75.03    70.07    60.00  
   832x624       74.55  
   800x600       72.19    75.00    60.32    56.25  
   640x480       75.00    72.81    66.67    59.94  
   720x400       70.08  
VIRTUAL1 connected 2048x1536+1920+1024 (normal left inverted right x axis y axis) 0mm x 0mm
   2048x1536_60.00  60.00*