Simplify bounds clipping a tiny bit

internal/scenario/draw.go

@@ -22,6 +22,17 @@ type IsoPt struct {
 func (s *Scenario) Update(screenX, screenY int) error {
 	s.tick += 1
 
+	x, y := ebiten.CursorPosition()
+	screenPos := CartPt{
+		X: float64(s.Viewpoint.X + x),
+		Y: float64(s.Viewpoint.Y + y),
+	}
+
+	s.selectedCell = screenPos.ToISO()
+
+	// TODO: zoom support will need a camera
+	// FIXME: adjust for Z level
+
 	return nil
 }
 
@@ -33,13 +44,15 @@ func (s *Scenario) Draw(screen *ebiten.Image) error {
 
 	sw, sh := screen.Size()
 
-	topLeft := CartPt{X: float64(s.Viewpoint.X), Y: float64(s.Viewpoint.Y)}.ToISO()
-	topLeft.X -= 5 // Ensure we paint to every visible section of the screeen.
-	topLeft.X -= 5 // FIXME: haxxx
+	topLeft := CartPt{
+		X: float64(s.Viewpoint.X - 2*cellWidth), // Ensure all visible cells are rendered
+		Y: float64(s.Viewpoint.Y - 2*cellHeight),
+	}.ToISO()
 
-	bottomRight := CartPt{X: float64(s.Viewpoint.X + sw), Y: float64(s.Viewpoint.Y + sh)}.ToISO()
-	bottomRight.X += 5
-	bottomRight.Y += 5
+	bottomRight := CartPt{
+		X: float64(s.Viewpoint.X + sw + 2*cellHeight),
+		Y: float64(s.Viewpoint.Y + sh + 5*cellHeight), // Z dimension requires it
+	}.ToISO()
 
 	// X+Y is constant for all tiles in a column
 	// X-Y is constant for all tiles in a row