fix: allow changing value in value-changed listener

fix #651

The else-if added at line 886 prevents duplicate events.

src/vaadin-combo-box-mixin.html

@@ -707,6 +707,11 @@
         const value = this._getItemValue(selectedItem);
         if (this.value !== value) {
           this.value = value;
+          if (this.value !== value) {
+            // The value was changed to something else in value-changed listener,
+            // so prevent from resetting it to the previous value.
+            return;
+          }
         }
 
         this._updateHasValue(true);
@@ -878,9 +883,7 @@
       if (this.opened) {
         this._focusedIndex = this.filteredItems.indexOf(e.detail.item);
         this.close();
-      }
-
-      if (this.selectedItem !== e.detail.item) {
+      } else if (this.selectedItem !== e.detail.item) {
         this.selectedItem = e.detail.item;
         this._detectAndDispatchChange();
       }

test/selecting-items.html

@@ -174,6 +174,44 @@
       expect(newComboBox.value).to.equal('foo');
     });
 
+    it('should allow changing the value in value-changed listener', done => {
+      combobox.open();
+      const items = combobox.$.overlay._selector.querySelectorAll('vaadin-combo-box-item');
+
+      combobox.addEventListener('value-changed', () => {
+        if (combobox.value === 'foo') {
+          combobox.value = 'bar';
+          setTimeout(() => {
+            expect(combobox.value).to.eql('bar');
+            expect(combobox.selectedItem).to.eql('bar');
+            expect(items[0].selected).to.be.false;
+            expect(items[1].selected).to.be.true;
+            done();
+          });
+        }
+      });
+      items[0].click();
+    });
+
+    it('should allow clearing the value in value-changed listener', done => {
+      combobox.open();
+      const item = combobox.$.overlay._selector.querySelector('vaadin-combo-box-item');
+
+      combobox.addEventListener('value-changed', () => {
+        if (combobox.value) {
+          combobox.value = '';
+          setTimeout(() => {
+            expect(combobox.value).to.eql('');
+            expect(combobox.selectedItem).not.to.be.ok;
+            expect(item.selected).to.be.false;
+            expect(combobox.hasAttribute('has-value')).to.be.false;
+            done();
+          });
+        }
+      });
+      item.click();
+    });
+
     describe('`change` event', () => {
       it('should fire `value-changed` and `selected-item-changed` before `changed`', done => {
         combobox.addEventListener('change', () => {