Check the distance between the point clicked and each Line on the form. If the closest Line is within 60 twips of the mouse, select that Line.

' Calculate the distance between the point and the segment.
Private Function DistToSegment(ByVal px As Single, ByVal py _
    As Single, ByVal X1 As Single, ByVal Y1 As Single, _
    ByVal X2 As Single, ByVal Y2 As Single) As Single
Dim dx As Single
Dim dy As Single
Dim t As Single

    dx = X2 - X1
    dy = Y2 - Y1
    If dx = 0 And dy = 0 Then
        ' It's a point not a line segment.
        dx = px - X1
        dy = py - Y1
        DistToSegment = Sqr(dx * dx + dy * dy)
        Exit Function
    End If

    t = (px + py - X1 - Y1) / (dx + dy)

    If t < 0 Then
        dx = px - X1
        dy = py - Y1
    ElseIf t > 1 Then
        dx = px - X2
        dy = py - Y2
    Else
        X2 = X1 + t * dx
        Y2 = Y1 + t * dy
        dx = px - X2
        dy = py - Y2
    End If
    DistToSegment = Sqr(dx * dx + dy * dy)
End Function

' See if the user clicked on a segment.
Private Sub Form_MouseUp(Button As Integer, Shift As _
    Integer, X As Single, Y As Single)
' Maximum distance that counts as a hit.
Const MAX_HIT_DIST = 60

Dim i As Integer
Dim best_i As Integer
Dim best_dist As Single
Dim dist As Single

    ' Deselect the previously selected segment.
    If m_SelectedSegment >= 0 Then
        With Line1(m_SelectedSegment)
            .BorderColor = vbBlack
            .BorderWidth = 1
        End With
        m_SelectedSegment = -1
    End If

    ' See which segment is closest to the mouse.
    best_dist = MAX_HIT_DIST + 1
    For i = Line1.LBound To Line1.UBound
        With Line1(i)
            dist = DistToSegment(X, Y, .X1, .Y1, .X2, .Y2)
            If dist < best_dist Then
                best_dist = dist
                best_i = i
            End If
        End With
    Next i

    ' See if this is close enough.
    If best_dist <= MAX_HIT_DIST Then
        m_SelectedSegment = best_i
        With Line1(m_SelectedSegment)
            .BorderColor = vbRed
            .BorderWidth = 3
        End With
    End If
End Sub