support formmethod

index.js

@@ -154,6 +154,32 @@ function isConnected(element) {
   return element.isConnected || document.body.contains(element);
 }
 
+/**
+ * @param {!Event} event
+ */
+function findFormSubmitter(event) {
+  if (event.submitter) {
+    return event.submitter;
+  }
+
+  var form = event.target;
+  if (!(form instanceof HTMLFormElement)) {
+    return null;
+  }
+
+  var submitter = dialogPolyfill.formSubmitter;
+  if (!submitter) {
+    var target = event.target;
+    var root = ('getRootNode' in target && target.getRootNode() || document);
+    submitter = root.activeElement;
+  }
+
+  if (submitter.form !== form) {
+    return null;
+  }
+  return submitter;
+}
+
 /**
  * @param {!Event} event
  */
@@ -162,32 +188,28 @@ function maybeHandleSubmit(event) {
     return;
   }
   var form = /** @type {!HTMLFormElement} */ (event.target);
-  if (!isFormMethodDialog(form)) {
-    return;
-  }
-  event.preventDefault();
 
   // We'd have a value if we clicked on an imagemap.
   var value = dialogPolyfill.useValue;
-  var submitter = dialogPolyfill.formSubmitter || event.submitter;
-  if (!value) {
-    if (!submitter) {
-      var root = ('getRootNode' in form && form.getRootNode() || document);
-      var activeElement = root.activeElement;
-      if (activeElement) {
-        submitter = event.submitter = activeElement;
-      }
-    }
-    if (submitter) {
-      value = submitter.value;
-    }
+  var submitter = findFormSubmitter(event);
+  if (value === null && submitter) {
+    value = submitter.value;
   }
 
-  const dialog = findNearestDialog(form);
+  // There should always be a dialog as this handler is added specifically on them, but check just
+  // in case.
+  var dialog = findNearestDialog(form);
   if (!dialog) {
     return;
   }
 
+  // Prefer formmethod on the button.
+  var formmethod = submitter && submitter.getAttribute('formmethod') || form.getAttribute('method');
+  if (formmethod !== 'dialog') {
+    return;
+  }
+  event.preventDefault();
+
   if (submitter) {
     dialog.close(value);
   } else {
@@ -795,6 +817,24 @@ if (window.HTMLDialogElement === undefined) {
 
   }, false);
 
+  /**
+   * Global 'submit' handler. This handles submits of `method="dialog"` which are invalid, i.e.,
+   * outside a dialog. They get prevented.
+   */
+  document.addEventListener('submit', function(ev) {
+    var form = ev.target;
+    var dialog = findNearestDialog(form);
+    if (dialog) {
+      return;  // ignore, handle there
+    }
+
+    var submitter = findFormSubmitter(ev);
+    var formmethod = submitter && submitter.getAttribute('formmethod') || form.getAttribute('method');
+    if (formmethod === 'dialog') {
+      ev.preventDefault();
+    }
+  });
+
   /**
    * Replace the native HTMLFormElement.submit() method, as it won't fire the
    * submit event and give us a chance to respond.

suite.js

@@ -14,7 +14,6 @@
  * the License.
  */
 
-
 void function() {
 
   /**
@@ -789,7 +788,10 @@ void function() {
       dialog.showModal();
 
       iframe.addEventListener('load', function() {
-        assert.fail('should not load a new page');
+        var href = iframe.contentWindow.location.href;
+        if (href !== 'about:blank') {
+          assert.fail('should not load a new page: ' + href);
+        }
       });
       button.click();
 
@@ -801,6 +803,69 @@ void function() {
         done();
       }, 50);
     });
+    test('form submit with formmethod', function(done) {
+      const iframeName = 'formmethod_test_frame';
+
+      var iframe = document.createElement('iframe');
+      iframe.setAttribute('name', iframeName);
+      document.body.append(iframe);
+      cleanup(iframe);
+
+      var form = document.createElement('form');
+      form.setAttribute('method', 'dialog');
+      form.setAttribute('target', iframeName);
+      form.setAttribute('action', '/test-invalid.html');
+      dialog.append(form);
+      dialog.show();
+
+      var button = document.createElement('button');
+      button.setAttribute('formmethod', 'get');
+      form.append(button);
+      button.value = '123';
+      button.textContent = 'Long button';
+
+      var timeout = window.setTimeout(function() {
+        assert.fail('page should load (form submit with formmethod)');
+      }, 500);
+      iframe.addEventListener('load', function() {
+        window.clearTimeout(timeout);
+        assert.isTrue(dialog.open);
+        done();
+      });
+      button.click();
+    });
+    test('form method="dialog" prevented outside dialog', function(done) {
+      const iframeName = 'outside_dialog_test';
+
+      var iframe = document.createElement('iframe');
+      iframe.setAttribute('name', iframeName);
+      document.body.append(iframe);
+      cleanup(iframe);
+
+      var form = document.createElement('form');
+      form.setAttribute('method', 'dialog');
+      form.setAttribute('target', iframeName);
+      form.setAttribute('action', '/test-invalid.html');
+      document.body.append(form);
+      cleanup(form);
+
+      iframe.addEventListener('load', function() {
+        if (iframe.contentWindow.location.href !== 'about:blank') {
+          assert.fail('should not load a new page: ' + iframe.contentWindow.location.href);
+        }
+      });
+
+      form.submit();
+
+      // Try with an actual button, too.
+      var button = document.createElement('button');
+      form.append(button);
+      button.click();
+
+      var timeout = window.setTimeout(function() {
+        done();
+      }, 1000);
+    });
   });
 
   suite('order', function() {