Fixed Area drawing assuming the first pixel of the *image.NRGBA is at i.Pix[0].

area.go

@@ -315,3 +315,12 @@ func (a *Area) setRect(x int, y int, width int, height int, rr *[]resizerequest)
 func (a *Area) preferredSize() (width int, height int) {
 	return a.sysData.preferredSize()
 }
+
+// internal function, but shared by all system implementations: &img.Pix[0] is not necessarily the first pixel in the image
+func pixelDataPos(img *image.NRGBA) int {
+	return img.PixOffset(img.Rect.Min.X, img.Rect.Min.Y)
+}
+
+func pixelData(img *image.NRGBA) *uint8 {
+	return &img.Pix[pixelDataPos(img)]
+}

area_darwin.go

@@ -92,7 +92,7 @@ func areaView_drawRect(self C.id, rect C.struct_xrect) {
 	}
 	i := s.handler.Paint(cliprect)
 	C.drawImage(
-		unsafe.Pointer(&i.Pix[0]), C.int64_t(i.Rect.Dx()), C.int64_t(i.Rect.Dy()), C.int64_t(i.Stride),
+		unsafe.Pointer(pixelData(i)), C.int64_t(i.Rect.Dx()), C.int64_t(i.Rect.Dy()), C.int64_t(i.Stride),
 		C.int64_t(cliprect.Min.X), C.int64_t(cliprect.Min.Y))
 }
 

area_unix.go

@@ -60,7 +60,7 @@ func our_area_draw_callback(widget *C.GtkWidget, cr *C.cairo_t, data C.gpointer)
 	// pixel order is [R G B A] (see Example 1 on https://developer.gnome.org/gdk-pixbuf/2.26/gdk-pixbuf-The-GdkPixbuf-Structure.html) so we don't have to convert anything
 	// gdk-pixbuf is not alpha-premultiplied (thanks to desrt in irc.gimp.net/#gtk+)
 	pixbuf := C.gdk_pixbuf_new_from_data(
-		(*C.guchar)(unsafe.Pointer(&i.Pix[0])),
+		(*C.guchar)(unsafe.Pointer(pixelData(i))),
 		C.GDK_COLORSPACE_RGB,
 		C.TRUE,			// has alpha channel
 		8,				// bits per sample

area_windows.go

@@ -109,7 +109,7 @@ func paintArea(s *sysData) {
 	// we don't have a choice but to convert it ourselves
 	// TODO make realbits a part of sysData to conserve memory
 	realbits := make([]byte, 4 * i.Rect.Dx() * i.Rect.Dy())
-	p := 0
+	p := pixelDataPos(i)
 	q := 0
 	for y := i.Rect.Min.Y; y < i.Rect.Max.Y; y++ {
 		nextp := p + i.Stride

todo.md

@@ -71,7 +71,6 @@ super ultra important things:
 - GTK+ indefinite progress bar animation is choppy: make sure the speed we have now is the conventional speed for GTK+ programs (HIG doesn't list any) and that the choppiness is correct
 - Message boxes are not application-modal on some platforms
 - cast all objc_msgSend() direct invocations to the approrpiate types; this is how you're supposed to do things: https://developer.apple.com/library/ios/documentation/General/Conceptual/CocoaTouch64BitGuide/ConvertingYourAppto64-Bit/ConvertingYourAppto64-Bit.html http://lists.apple.com/archives/objc-language/2014/Jan/msg00011.html http://lists.apple.com/archives/cocoa-dev/2006/Feb/msg00753.html and many others
-- Area drawing assumes that i.Pix[0] is the first pixel; this might not be the case (and the current documentation for AreaHandler.Paint() allows it). Fix it.
 
 other things:
 - on windows 7, progress bars seem to animate from 0 -> pos when you turn off marquee mode and set pos; see if that's documented or if I'm doing something wrong