ili9341: st7789: fix SetScrollArea

The existing code was broken in a few ways:

  - It didn't use the correct operator precedence for the VSCRDEF VSA
    variable: it needed some extra parentheses to be correct.
  - It used the configured height instead of the actual display height
    for calculating VSA, which is incorrect. TFA+VSA+BFA must always be
    exactly 320, even if a lower value is configured.
  - If a lower than 320 pixel height is configured, the bottomFixedArea
    parameter applied to the whole 320 pixel screen height. Because this
    seems counter intuitive (and relies on properties of any given
    screen), I've changed it to work from the actual visible bottom of
    the screen (which may be smaller than 320 pixels).
    TODO: this doesn't take RowOffset into account, while it probably
    should.

I haven't fixed the st7735 implementation, because I didn't have example
code on hand that would easily work on a st7735 screen. This is left as
a TODO for the future.

ili9341/ili9341.go

@@ -320,10 +320,16 @@ func (d *Device) SetRotation(rotation drivers.Rotation) error {
 // SetScrollArea sets an area to scroll with fixed top/bottom or left/right parts of the display
 // Rotation affects scroll direction
 func (d *Device) SetScrollArea(topFixedArea, bottomFixedArea int16) {
+	if d.height < 320 {
+		// The screen doesn't use the full 320 pixel height.
+		// Enlarge the bottom fixed area to fill the 320 pixel height, so that
+		// bottomFixedArea starts from the visible bottom of the screen.
+		bottomFixedArea += 320 - d.height
+	}
 	cmdBuf[0] = uint8(topFixedArea >> 8)
 	cmdBuf[1] = uint8(topFixedArea)
-	cmdBuf[2] = uint8(d.height - topFixedArea - bottomFixedArea>>8)
-	cmdBuf[3] = uint8(d.height - topFixedArea - bottomFixedArea)
+	cmdBuf[2] = uint8((320 - topFixedArea - bottomFixedArea) >> 8)
+	cmdBuf[3] = uint8(320 - topFixedArea - bottomFixedArea)
 	cmdBuf[4] = uint8(bottomFixedArea >> 8)
 	cmdBuf[5] = uint8(bottomFixedArea)
 	d.sendCommand(VSCRDEF, cmdBuf[:6])

st7735/st7735.go

@@ -235,6 +235,8 @@ func (d *Device) setWindow(x, y, w, h int16) {
 
 // SetScrollWindow sets an area to scroll with fixed top and bottom parts of the display
 func (d *Device) SetScrollArea(topFixedArea, bottomFixedArea int16) {
+	// TODO: this code is broken, see the st7789 and ili9341 implementations for
+	// how to do this correctly.
 	d.Command(VSCRDEF)
 	d.Tx([]uint8{
 		uint8(topFixedArea >> 8), uint8(topFixedArea),

st7789/st7789.go

@@ -554,9 +554,16 @@ func (d *Device) IsBGR(bgr bool) {
 
 // SetScrollArea sets an area to scroll with fixed top and bottom parts of the display.
 func (d *Device) SetScrollArea(topFixedArea, bottomFixedArea int16) {
+	if d.height < 320 {
+		// The screen doesn't use the full 320 pixel height.
+		// Enlarge the bottom fixed area to fill the 320 pixel height, so that
+		// bottomFixedArea starts from the visible bottom of the screen.
+		bottomFixedArea += 320 - d.height
+	}
+	verticalScrollArea := 320 - topFixedArea - bottomFixedArea
 	copy(d.buf[:6], []uint8{
 		uint8(topFixedArea >> 8), uint8(topFixedArea),
-		uint8(d.height - topFixedArea - bottomFixedArea>>8), uint8(d.height - topFixedArea - bottomFixedArea),
+		uint8(verticalScrollArea >> 8), uint8(verticalScrollArea),
 		uint8(bottomFixedArea >> 8), uint8(bottomFixedArea)})
 	d.startWrite()
 	d.sendCommand(VSCRDEF, d.buf[:6])