Repair spacebar textInput

Browsers that natively support the `textInput` event appear to have a bug: when preventing default behavior for a `textInput` event occurring for a spacebar keypress, the character is prevented from being inserted **but the browser scrolls down**.

Minimal repro example: http://jsfiddle.net/salier/bX4fw/

This is ridiculous, since scrolling makes no sense when the user is focused in a textinput or contenteditable. Preventing default at the `textInput` stage should mean to prevent the character from being inserted, and should have no impact at all on scrolling behavior. I have filed this as a Chromium bug (https://code.google.com/p/chromium/issues/detail?id=355103) but I'm not going to hold out much hope that they'll fix it.

To resolve this, I'm special-casing the spacebar character at the plugin level, in `BeforeInputEventPlugin`. I looked for ways to do this at the component level, but it seems to me that this is simply a browser bug and it's cleaner to handle it there.

In browsers that can use the native `textInput` event, I'm checking the code of the pressed key. If it's the spacebar, we dispatch the synthetic event as if there were no native `textInput` event -- as if we were running Firefox. Then, if the synthetic event is not canceled and we make it through to the native `textInput` event, bail if the character data is a space character.

src/browser/eventPlugins/BeforeInputEventPlugin.js

@@ -26,12 +26,15 @@ var SyntheticInputEvent = require('SyntheticInputEvent');
 
 var keyOf = require('keyOf');
 
-var useBeforeInputEvent = (
+var canUseTextInputEvent = (
   ExecutionEnvironment.canUseDOM &&
   'TextEvent' in window &&
   !('documentMode' in document)
 );
 
+var SPACEBAR_CODE = 32;
+var SPACEBAR_CHAR = String.fromCharCode(SPACEBAR_CODE);
+
 var topLevelTypes = EventConstants.topLevelTypes;
 
 // Events and their corresponding property names.
@@ -100,13 +103,48 @@ var BeforeInputEventPlugin = {
 
     var chars;
 
-    if (useBeforeInputEvent) {
-      // For browsers that support `textInput` events natively, don't do
-      // anything with keypress, composition, etc.
-      if (topLevelType !== topLevelTypes.topTextInput) {
-        return;
+    if (canUseTextInputEvent) {
+      switch (topLevelType) {
+        case topLevelTypes.topKeyPress:
+          /**
+           * If native `textInput` events are available, our goal is to make
+           * use of them. However, there is a special case: the spacebar key.
+           * In Webkit, preventing default on a spacebar `textInput` event
+           * cancels character insertion, but it *also* causes the browser
+           * to fall back to its default spacebar behavior of scrolling the
+           * page.
+           *
+           * Tracking at:
+           * https://code.google.com/p/chromium/issues/detail?id=355103
+           *
+           * To avoid this issue, use the keypress event as if no `textInput`
+           * event is available.
+           */
+          var which = nativeEvent.which;
+          if (which !== SPACEBAR_CODE) {
+            return;
+          }
+
+          chars = String.fromCharCode(which);
+          break;
+
+        case topLevelTypes.topTextInput:
+          // Record the characters to be added to the DOM.
+          chars = nativeEvent.data;
+
+          // If it's a spacebar character, assume that we have already handled
+          // it at the keypress level and bail immediately.
+          if (chars === SPACEBAR_CHAR) {
+            return;
+          }
+
+          // Otherwise, carry on.
+          break;
+
+        default:
+          // For other native event types, do nothing.
+          return;
       }
-      chars = nativeEvent.data;
     } else {
       switch (topLevelType) {
         case topLevelTypes.topPaste: