[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 by
default.

This change fixes the behavior of GC#copyArea() by explicitly making
Cairo first pipe the copied area into an intermediate surface to be
painted into the actual target area afterwards. 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

@@ -548,7 +548,13 @@ void copyAreaInPixels(int srcX, int srcY, int width, int height, int destX, int
 		Cairo.cairo_set_source_surface(handle, data.image.surface, deltaX, deltaY);
 		Cairo.cairo_rectangle(handle, destX, destY, width, height);
 		Cairo.cairo_set_operator(handle, Cairo.CAIRO_OPERATOR_SOURCE);
+		// As source and target area may be overlapping, we need to draw on
+		// an intermediate surface to avoid that parts of the source area are
+		// overwritten before reading it to be copied
+		Cairo.cairo_push_group(handle);
 		Cairo.cairo_fill(handle);
+		Cairo.cairo_pop_group_to_source(handle);
+		Cairo.cairo_paint(handle);
 	} 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);