I think the problem is probably what you said "You compute on Real domain, but the line live on Integer ( pixels ) domain". I'm still looking into this, your code certainly helps in debugging it, thanks.

>>Yes, it is odd that certain black points are being logged to the debug output window. They shouldn't be. Perhaps it is a paint problem? I haven't found any logical errors in the code, nor have any been reported by users. Any thoughts?
>>
>
>It is not so simple.
>
>First point:
>Add OBJTOCLIENT() for hierarchy support
>
>The problems:
>You compute on Real domain, but the line live on Integer ( pixels ) domain,
>then:
>1. VFP Rounding is ?
>2. BorderWidth is the line width, but you use a 2*BorderWidth zone
>
>This code is more precise for detect fails,
>and if you resize the form to 2000X1000 pixels you can see it better.
>On upper left corner you have many FALSE NEGATIVE,
>On bottom righ corner you have many FALSE POSITIVE.
>
>

>PUBLIC oform1
>ACTIVATE WINDOW "debug output"
>
>oform1=NEWOBJECT("form1")
>oform1.Show
>RETURN
>
>DEFINE CLASS form1 AS form
>
>	DoCreate = .T.
>	Caption = "Form1"
>	Name = "Form1"
>	DESKTOP = .T.
>	
>	ADD OBJECT line1 AS lineex WITH ;
>		ANCHOR = 15,;
>		BorderWidth = 3, ;
>		Height = THISFORM.height - 4, ;
>		Left = 4, ;
>		Top = 4, ;
>		Width = THISFORM.Width - 4, ;
>		Name = "Line1"
>ENDDEFINE
>
>DEFINE CLASS lineex as Line
>
>	* this is a my wish routine for every visual baseclass, but VFPT ......
>	PROCEDURE ObjectHitTest(nXCoord,nYCoord)
>		IF		BETWEEN(m.nXCoord-OBJTOCLIENT(m.This,OBJTOCLIENT_LEFT)	,0,OBJTOCLIENT(m.This,OBJTOCLIENT_WIDTH) -1);
>			AND BETWEEN(m.nYCoord-OBJTOCLIENT(m.This,OBJTOCLIENT_TOP)	,0,OBJTOCLIENT(m.This,OBJTOCLIENT_HEIGHT)-1)
>			* check line
>			* problem : Line is transparent !!!!
>			*
>			IF thisform.Point(m.nXCoord, m.nYCoord)=m.This.BorderColor
>				RETURN 1
>			ENDIF
>		ENDIF
>		* none
>		RETURN 0
>	
>	* the ObjectHitTest way
>	PROCEDURE MouseMove
>		PARAMETERS nButton, nShift, nXCoord, nYCoord
>		*!* Is mouse directly over the line?
>		*!* if so, color it red
>		IF this.ObjectHitTest(nXCoord, nYCoord)
>			this.BorderColor = RGB(255,0,0)
>		ELSE
>			this.BorderColor = RGB(0,0,0)
>		ENDIF
>		IF thisform.Point(m.nXCoord, m.nYCoord)=0
>			DEBUGOUT m.nXCoord, m.nYCoord
>		ENDIF	
>	ENDPROC
>	
>	* your code
>	PROCEDURE MouseMove
>		PARAMETERS nButton, nShift, nXCoord, nYCoord
>		LOCAL lnFullX, lnFullY, lnCurrentX
>		lnFullX = this.Width
>		lnFullY = this.height
>		lnCurrentXPercentage = (nXCoord - this.Left)/lnFullX
>
>		IF This.LineSlant = "/"
>			lnCurrentXPercentage = (1 - lnCurrentXPercentage)
>		ENDIF
>		*!* Is mouse directly over the line?
>		*!* if so, color it red
>		lineDetect= BETWEEN((nYCoord - this.Top) / lnCurrentXPercentage, lnFullY - this.BorderWidth, lnFullY + this.BorderWidth)
>        DO CASE
>        	CASE lineDetect AND Thisform.Point(m.nXCoord, m.nYCoord)#0
>        		DEBUGOUT "FALSE POSITIVE",m.nXCoord, m.nYCoord
>        	CASE !lineDetect AND Thisform.Point(m.nXCoord, m.nYCoord)=0
>        		DEBUGOUT "FALSE NEGATIVE",m.nXCoord, m.nYCoord
>		ENDCASE
>		
>	ENDPROC
>
>ENDDEFINE
>