Ensure extend op stays sequential when copying px #3928

test/unit/extend.js

@@ -73,6 +73,20 @@ describe('Extend', function () {
         });
     });
 
+    it('extend top with mirroring uses ordered read', async () => {
+      const data = await sharp(fixtures.inputJpg)
+        .extend({
+          extendWith: 'mirror',
+          top: 1
+        })
+        .png({ compressionLevel: 0 })
+        .toBuffer();
+
+      const { width, height } = await sharp(data).metadata();
+      assert.strictEqual(2725, width);
+      assert.strictEqual(2226, height);
+    });
+
     it(`extend sides unequally with RGBA (${extendWith})`, function (done) {
       sharp(fixtures.inputPngWithTransparency16bit)
         .resize(120)