Inconsistent position (degrees) after warping

Coding
1

Hello,
I’m trying to figure out how to use the warper object and I’m currently facing an issue running this piece of code:

from psychopy import visual, monitors, event
from psychopy.visual.windowwarp import Warper

warp = True

mon = monitors.Monitor('tryMonitor')
mon.setDistance(20)

stimWin = visual.Window(
    [1920,1200],screen = 0,fullscr = True,
    monitor = mon, units = 'degFlat',
    useFBO = True,
    allowStencil = True,
    )

monitorWidthDeg = 104.8

gratingBack = visual.GratingStim(
    stimWin,tex = 'sqr', mask = None, units = 'degFlat',
    maskParams = None, pos = (0.0, 0.0), 
    
    size = [monitorWidthDeg, 10], 
    
    sf = 1,
    ori = 0.0, phase = (0.0, 0.0), texRes = 128,contrast = 0.9,
    opacity = 1.0,autoDraw = False
    )

if warp:
    warper = Warper(
        stimWin,
        warp = 'spherical',
        warpGridsize = 300,
        eyepoint = [0.5, 0.5]
        )
    warper.dist_cm = 20
    warper.changeProjection('spherical')


keepGoing = True

event.clearEvents('keyboard')
while keepGoing:
    if len(event.getKeys(keyList=('escape','q'))) > 0:
        event.clearEvents('keyboard')
        keepGoing = False
        break
    gratingBack.draw()
    stimWin.flip()

I know my monitor width in degrees, and I can confirm it because, by not using the warper, I generate a stimulus like this one:

img1
img11920×1200 9.43 KB

Moreover, i know the width is correct because if I reduce the size of the grating to 99% the original size, I get this stimulus:

img2
img21920×1200 9.44 KB

However, when applying the warper, the size of the warped stimulus is extended beyond the limits of my screen. Now I need to downscale the size of the grating to 80% to be able to see the entire stimulus:

img4
img41920×1200 72.3 KB

This raises a problem: If you specify the position or the size of a stimulus in degrees, its actual position on the monitor is not physically accurate anymore after the warping. Am I missing something or is this the expected behavior?

is it correct to set the units to degFlat in this instance?