[BUG] Config Panel does not allow negative numbers for X_MIN_POS or Y_MIN_POS in the Geometry section of Configuration.h

[bigjeff5]

For some reason negative numbers are sanitized to positive numbers in these fields. The only way to set these values to negative is to set them in the raw Configuration.h file and not touch the values in the AutoBuildMarlin panel, else they will be set to positive.

Negative numbers are essential when your endstops are outside the build volume. When set to 0, Marlin thinks the build plate is right under the nozzle at the home position, but in many gantry style printers (and possibly other styles) the nozzle will be 15-30mm off the build plate at 0,0. ABL then fails, large prints can fail, etc.

I've seen no other place to set this offset than here, and all the information I've found says to do this in this value, so I don't understand why AutoBuildMarlin is forcing this to a positive value, when in many cases it will NEED to be negative*.

M206 can be used to soft set this value on a running system, but commands like G29 do not respect M206 settings and will miss the bed when trying to probe.

*Note: it does make sense to force this to positive for the Z axis, because anything below 0 will cause your nozzle to crash the bed unless the nozzel is also outside the x/y boundary of the bed. At the same time, bed leveling would properly correct for a negative value, so maybe there are cases where a negative Z value makes sense as well.