Do not assign node.value on input creation if no change will occur (#12925)

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