Stability issue with mouse controlled overlay

Some while ago Krzysztof Jankowski created a very simple but strangely additive game (name unknown). A vertical moving line from the top of the screen had to be stopped from touching a horizontal rectangle at the bottom of the screen. Another overlayed rectangular cursor could be moved horizontally across the screen with the mouse. If you held down the left-hand mouse button at the same time an orange wash was drawn from the cursor position until it covered the vertical line (in effect eliminating it). Subsequent lines followed.

Written for MODE 28, I had a go at converting it for a 1080p full HD monitor. Unfortunately the overlay is unstable spoiling the effect. Is there a cure? The mouse overlay is shown below…

MODE 1920,1080,32
PX%=0
PY%=0
REPEAT
GCOL 0,0,255
RECTANGLE FILL 0,0,3838,100
MOUSE MX%,MY%,MB%
GCOL 255,0,0
RECTANGLE FILL PX%-50,PY%,100
IF MX%<PX% THEN : PX%=PX%-1 : ENDIF
IF MX%>PX% THEN : PX%=PX%+1 : ENDIF
UNTIL0

Is there a specific effect display?

What machine is this running on? Various machines may handle the mouse overlay differently.