[GTK] Fix GC#copyArea() for overlapping source and target areas #1756

The GC provides a #copyArea() method that copies an area inside the
surface of the GC to another area in the same surface. When using a
surface directly created via the Cairo API (or with the GDK API on
Wayland), this operation is broken.
When the source and target area are overlapping and the source is
above-left of the target area, the source area will appear replicated in
the target area, as Cairo does not perform any buffering but does a
linewise/chunkwise write from the source area to the target area.

This change fixes the behavior of GC#copyArea() by explicitly buffering
the area to be copied. It also removes the existing workaround in images
that avoids the instantiation of a surface via the Cairo API, as the
reason for that workaround was the issue fixed by this change. It
improves the behavior of consumers of that functionality on GTK4 and
Wayland. An according regression test is added.

Fixes #1756

Author: HeikoKlare

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/graphics/GC.java

@@ -545,10 +545,25 @@ void copyAreaInPixels(int srcX, int srcY, int width, int height, int destX, int
 	if (deltaX == 0 && deltaY == 0) return;
 	long drawable = data.drawable;
 	if (data.image != null) {
-		Cairo.cairo_set_source_surface(handle, data.image.surface, deltaX, deltaY);
+		// When filling a part of a surface with another part of a surface, Cairo does not buffer
+		// the whole data to copy but performs some chunk-wise copy from the top left to the bottom right.
+		// Thus if the source and the target area are overlapping, Cairo may copy a chunk into a target
+		// area that is then copied on later, leading to unwanted replications of that source chunk in
+		// the target area.
+		// To avoid this, we buffer the copied area in a separate surface.
+		int imageFormat = Cairo.cairo_image_surface_get_format(data.image.surface);
+		long tempSurface = Cairo.cairo_image_surface_create(imageFormat, width, height);
+		long tempGC = Cairo.cairo_create(tempSurface);
+		Cairo.cairo_set_source_surface(tempGC, data.image.surface, -srcX, -srcY);
+		Cairo.cairo_rectangle(tempGC, 0, 0, width, height);
+		Cairo.cairo_set_operator(tempGC, Cairo.CAIRO_OPERATOR_SOURCE);
+		Cairo.cairo_fill(tempGC);
+		Cairo.cairo_destroy(tempGC);
+		Cairo.cairo_set_source_surface(handle, tempSurface, destX, destY);
 		Cairo.cairo_rectangle(handle, destX, destY, width, height);
 		Cairo.cairo_set_operator(handle, Cairo.CAIRO_OPERATOR_SOURCE);
 		Cairo.cairo_fill(handle);
+		Cairo.cairo_surface_destroy(tempSurface);
 	} else if (drawable != 0) {
 		Cairo.cairo_save(handle);
 		Cairo.cairo_rectangle(handle, destX, destY, width, height);

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/graphics/Image.java

@@ -1260,11 +1260,7 @@ void init(int width, int height) {
 	this.type = SWT.BITMAP;
 
 	/* Create the pixmap */
-	if (GTK.GTK4) {
-		surface = Cairo.cairo_image_surface_create(Cairo.CAIRO_FORMAT_RGB24, width, height);
-	} else {
-		surface = GDK.gdk_window_create_similar_surface(GDK.gdk_get_default_root_window(), Cairo.CAIRO_CONTENT_COLOR, width, height);
-	}
+	surface = Cairo.cairo_image_surface_create(Cairo.CAIRO_FORMAT_ARGB32, width, height);
 	if (surface == 0) SWT.error(SWT.ERROR_NO_HANDLES);
 	// When we create a blank image we need to set it to 100 in GTK3 as we draw using 100% scale.
 	// Cairo will take care of scaling for us when image needs to be scaled.

tests/org.eclipse.swt.tests/JUnit Tests/org/eclipse/swt/tests/junit/Test_org_eclipse_swt_graphics_GC.java

@@ -172,6 +172,52 @@ public void test_copyAreaIIIIII() {
 	assertEquals(":d:", whiteRGB, palette.getRGB(pixel));
 }
 
+@Test
+public void test_copyAreaIIIIII_overlapingSourceTarget() {
+	Color red= display.getSystemColor(SWT.COLOR_RED);
+	Color blue = display.getSystemColor(SWT.COLOR_BLUE);
+	RGB redRGB = getRealRGB(red);
+	RGB blueRGB = getRealRGB(blue);
+
+	gc.setBackground(red);
+	gc.fillRectangle(image.getBounds());
+	gc.setBackground(blue);
+	gc.fillRectangle(0, 100, 200, 100);
+
+	ImageData imageData = image.getImageData();
+	PaletteData palette = imageData.palette;
+
+	int pixel = imageData.getPixel(0, 0);
+	assertEquals(redRGB, palette.getRGB(pixel));
+	pixel = imageData.getPixel(0, 105);
+	assertEquals(blueRGB, palette.getRGB(pixel));
+	pixel = imageData.getPixel(0, 155);
+	assertEquals(blueRGB, palette.getRGB(pixel));
+
+	gc.copyArea(0, 50, 200, 100, 0, 100);
+
+	imageData = image.getImageData();
+	palette = imageData.palette;
+
+	if (DPIUtil.getDeviceZoom() != 100) {
+		//TODO Fix non integer scaling factors.
+		if (SwtTestUtil.verbose) {
+			System.out.println("Excluded test_copyAreaIIIIII(org.eclipse.swt.tests.junit.Test_org_eclipse_swt_graphics_GC)");
+		}
+		return;
+	}
+
+	pixel = imageData.getPixel(0, 105);
+	assertEquals(redRGB, palette.getRGB(pixel));
+	pixel = imageData.getPixel(0, 145);
+	assertEquals(redRGB, palette.getRGB(pixel));
+	pixel = imageData.getPixel(0, 155);
+	assertEquals(blueRGB, palette.getRGB(pixel));
+	pixel = imageData.getPixel(0, 195);
+	assertEquals(blueRGB, palette.getRGB(pixel));
+}
+
+
 @Test
 public void test_copyAreaLorg_eclipse_swt_graphics_ImageII() {
 	Color white = display.getSystemColor(SWT.COLOR_WHITE);