Do not assign node.value on input creation if no change will occur

This commit fixes an issue where assigning an empty string to required
text inputs triggers the invalid state in Firefox (~60.0.1).

It does this by first comparing the initial state value to the current
value property on the text element. This:

1. Prevents the validation issue
2. Avoids an extra DOM Mutation in some cases

fixtures/dom/src/components/fixtures/text-inputs/index.js

@@ -64,21 +64,36 @@ class TextInputFixtures extends React.Component {
           <Fixture>
             <form className="control-box">
               <fieldset>
-                <legend>Text</legend>
+                <legend>Empty value prop string</legend>
+                <input value="" required={true} />
+              </fieldset>
+              <fieldset>
+                <legend>No value prop</legend>
                 <input required={true} />
               </fieldset>
               <fieldset>
-                <legend>Date</legend>
+                <legend>Empty defaultValue prop string</legend>
+                <input required={true} defaultValue="" />
+              </fieldset>
+              <fieldset>
+                <legend>No value prop date input</legend>
                 <input type="date" required={true} />
               </fieldset>
+              <fieldset>
+                <legend>Empty value prop date input</legend>
+                <input type="date" value="" required={true} />
+              </fieldset>
             </form>
           </Fixture>
 
           <p className="footnote">
             Checking the date type is also important because of a prior fix for
             iOS Safari that involved assigning over value/defaultValue
-            properties of the input to prevent a display bug. This also
-            triggered input validation.
+            properties of the input to prevent a display bug. This also triggers
+            input validation.
+          </p>
+          <p className="footnote">
+            The date inputs should be blank in iOS. This is not a bug.
           </p>
         </TestCase>
 

packages/react-dom/src/client/ReactDOMFiberInput.js

@@ -209,16 +209,24 @@ export function postMountWrapper(element: Element, props: Object) {
   const node = ((element: any): InputWithWrapperState);
 
   if (props.hasOwnProperty('value') || props.hasOwnProperty('defaultValue')) {
+    const initialValue = '' + node._wrapperState.initialValue;
+    const currentValue = node.value;
+
     // Do not assign value if it is already set. This prevents user text input
     // from being lost during SSR hydration.
-    if (node.value === '') {
-      node.value = '' + node._wrapperState.initialValue;
+    if (currentValue === '') {
+      // Do not re-assign the value property if there is no change. This
+      // potentially avoids a DOM write and prevents Firefox (~60.0.1) from
+      // prematurely marking required inputs as invalid
+      if (initialValue !== currentValue) {
+        node.value = initialValue;
+      }
     }
 
     // value must be assigned before defaultValue. This fixes an issue where the
     // visually displayed value of date inputs disappears on mobile Safari and Chrome:
     // https://github.com/facebook/react/issues/7233
-    node.defaultValue = '' + node._wrapperState.initialValue;
+    node.defaultValue = initialValue;
   }
 
   // Normally, we'd just do `node.checked = node.checked` upon initial mount, less this bug