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Should HDMI config.txt overscan_* adjustments act independent of each other?

If not, what's happening? ... what would be the process to make predictable adjustments?

For example, if overscan_left & overscan_right horizontal values are held constant while overscan_top & overscan_bottom vertical values are adjusted, should the visual horizontal scan remain as-is?

In particular, the settings in config.txt below provide the correct horizontal presentation. However, the prompt line is mostly cut off at the bottom.

Any attempt which hold a working set of horizontal values constant in the config.txt file, while adjusting the only vertical values still causes the horizontal scan on the monitor to change.

Any attempt to shorten the vertical (i.e. to show the prompt) causes the supposedly unchanged horizontal lines to widen and get cut off the monitor.

config.txt

## disable automatic overscan 
disable_overscan=1

## Lilliput 619A native*1.25: 1280-(2*140)=1000; 720-(2*60)=600;
overscan_left=140
overscan_right=140
overscan_top=60
overscan_bottom=60

### Lilliput 619A 7" native*1.25=1000x600, 60Hz, [15:9]
hdmi_cvt=1000 600 60 6
hdmi_group=2
hdmi_mode=87

Board: Raspberry Pi 3 B+
OS: Ubuntu Server 18.04 ... not sure if OS matters since config.txt is likely applied before the OS starts up.
Monitor: Lilliput 619A 7" 800x480 native, 60Hz, [15:9]

So far, any of a matrix of many attempts ends with some exclusive-or like solution. Either the prompt has been hidden or the horizontal line been partially hidden.

In summary, are HDMI config.txt overscan_* adjustments expected act independent of each other? If not, what's the relationship between them?

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