Fix the flicker on originalFill image load

This is not pretty but what we have to do is return the visible portion pixel for pixel.
The image is immediately refreshed so the correct, full, image (with aspect applied) is then used.

Fixes fyne-io#1432

CHANGELOG.md

@@ -56,6 +56,7 @@ More detailed release notes can be found on the [releases page](https://github.c
 * Respect SelectEntry datta changes on refresh (#1462)
 * Incorrect SelectEntry dropdown button position (#1361)
 * don't allow both single and double tap events to fire (#1381)
+* Fix issue where long or tall images could jump on load (#1266, #1432)
 * Add missing raster support in software render
 * Respect GOOS/GOARCH in fyne command utilities
 * BSD support in build tools

internal/painter/image.go

@@ -55,21 +55,26 @@ func PaintImage(img *canvas.Image, c fyne.Canvas, width, height int) image.Image
 				aspect := float32(origW) / float32(origH)
 				viewAspect := float32(width) / float32(height)
 
-				texW, texH := width, height
-				if viewAspect > aspect {
-					texW = int(float32(height) * aspect)
-				} else if viewAspect < aspect {
-					texH = int(float32(width) / aspect)
-				}
-
-				icon.SetTarget(0, 0, float64(texW), float64(texH))
-				// this is used by our render code, so let's set it to the file aspect
-				aspects[img.Resource] = aspect
+				firstPassOriginal := false
 				// if the image specifies it should be original size we need at least that many pixels on screen
 				if img.FillMode == canvas.ImageFillOriginal {
-					checkImageMinSize(img, c, origW, origH)
+					if !checkImageMinSize(img, c, origW, origH) {
+						firstPassOriginal = true
+					}
+				}
+
+				texW, texH := width, height
+				if firstPassOriginal { // we draw pixel 1:1 so we can subimage it below to fit in the current bounds
+					texW, texH = origW, origH
+				} else {
+					if viewAspect > aspect {
+						texW = int(float32(height) * aspect)
+					} else if viewAspect < aspect {
+						texH = int(float32(width) / aspect)
+					}
 				}
 
+				icon.SetTarget(0, 0, float64(texW), float64(texH))
 				tex = image.NewNRGBA(image.Rect(0, 0, texW, texH))
 				scanner := rasterx.NewScannerGV(origW, origH, tex, tex.Bounds())
 				raster := rasterx.NewDasher(width, height, scanner)
@@ -79,6 +84,13 @@ func PaintImage(img *canvas.Image, c fyne.Canvas, width, height int) image.Image
 					fyne.LogError("SVG Render error:", err)
 					return nil
 				}
+
+				if firstPassOriginal { // crop out the currently visible portion to avoid flicker when it sizes later
+					return imageSubset(tex, width, height, img.ScaleMode)
+				}
+
+				// this is used by our render code, so let's set it to the file aspect
+				aspects[img.Resource] = aspect
 				svgCachePut(img.Resource, tex, width, height)
 			}
 
@@ -93,29 +105,42 @@ func PaintImage(img *canvas.Image, c fyne.Canvas, width, height int) image.Image
 			return nil
 		}
 		origSize := pixels.Bounds().Size()
-		// this is used by our render code, so let's set it to the file aspect
-		aspects[img] = float32(origSize.X) / float32(origSize.Y)
 		// if the image specifies it should be original size we need at least that many pixels on screen
 		if img.FillMode == canvas.ImageFillOriginal {
-			checkImageMinSize(img, c, origSize.X, origSize.Y)
+			if !checkImageMinSize(img, c, origSize.X, origSize.Y) {
+				return imageSubset(pixels, width, height, img.ScaleMode)
+			}
 		}
+		// this is used by our render code, so let's set it to the file aspect
+		aspects[img] = float32(origSize.X) / float32(origSize.Y)
 
 		return scaleImage(pixels, width, height, img.ScaleMode)
 	case img.Image != nil:
 		origSize := img.Image.Bounds().Size()
-		// this is used by our render code, so let's set it to the file aspect
-		aspects[img] = float32(origSize.X) / float32(origSize.Y)
 		// if the image specifies it should be original size we need at least that many pixels on screen
 		if img.FillMode == canvas.ImageFillOriginal {
-			checkImageMinSize(img, c, origSize.X, origSize.Y)
+			if !checkImageMinSize(img, c, origSize.X, origSize.Y) {
+				return imageSubset(img.Image, width, height, img.ScaleMode)
+			}
 		}
 
+		// this is used by our render code, so let's set it to the file aspect
+		aspects[img] = float32(origSize.X) / float32(origSize.Y)
 		return scaleImage(img.Image, width, height, img.ScaleMode)
 	default:
 		return image.NewNRGBA(image.Rect(0, 0, 1, 1))
 	}
 }
 
+// imageSubset returns a portion of an image based on pixel dimensions.
+// This is needed when we initially display an image with original dimensions that are still being calculated.
+func imageSubset(img image.Image, width, height int, scale canvas.ImageScale) image.Image {
+	sub := img.(interface {
+		SubImage(r image.Rectangle) image.Image
+	}).SubImage(image.Rect(0, 0, width, height))
+	return scaleImage(sub, width, height, scale)
+}
+
 func scaleImage(pixels image.Image, scaledW, scaledH int, scale canvas.ImageScale) image.Image {
 	pixW := fyne.Min(scaledW, pixels.Bounds().Dx()) // don't push more pixels than we have to
 	pixH := fyne.Min(scaledH, pixels.Bounds().Dy()) // the GL calls will scale this up on GPU.
@@ -145,11 +170,14 @@ func drawSVGSafely(icon *oksvg.SvgIcon, raster *rasterx.Dasher) error {
 	return err
 }
 
-func checkImageMinSize(img *canvas.Image, c fyne.Canvas, pixX, pixY int) {
+func checkImageMinSize(img *canvas.Image, c fyne.Canvas, pixX, pixY int) bool {
 	dpSize := fyne.NewSize(internal.UnscaleInt(c, pixX), internal.UnscaleInt(c, pixY))
 
 	if img.MinSize() != dpSize {
 		img.SetMinSize(dpSize)
 		canvas.Refresh(img) // force the initial size to be respected
+		return false
 	}
+
+	return true
 }