Issue #1509 - Invalidate previous result when datalist is changed.

Even if `<datalist>` is dynamically changed, the autocomplete controller still
uses the previous search result. If changed, we have to ignore the previous
result that may now be invalid.

Also, even if `<datalist>` is changed, we have to keep the selected index
(See Mozilla Bug 595069), so we cannot use `ResetInternalState` in this
situation because it resets the selected index.

This resolves #1509.

toolkit/components/autocomplete/nsAutoCompleteController.cpp

@@ -1183,11 +1183,16 @@ nsAutoCompleteController::BeforeSearches()
   mSearchStatus = nsIAutoCompleteController::STATUS_SEARCHING;
   mDefaultIndexCompleted = false;
 
-  // The first search result will clear mResults array, though we should pass
-  // the previous result to each search to allow them to reuse it.  So we
-  // temporarily cache current results till AfterSearches().
-  if (!mResultCache.AppendObjects(mResults)) {
-    return NS_ERROR_OUT_OF_MEMORY;
+  bool invalidatePreviousResult = false;
+  mInput->GetInvalidatePreviousResult(&invalidatePreviousResult);
+
+  if (!invalidatePreviousResult) {
+    // ClearResults will clear the mResults array, but we should pass the
+    // previous result to each search to allow reusing it.  So we temporarily
+    // cache the current results until AfterSearches().
+    if (!mResultCache.AppendObjects(mResults)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
   }
 
   mSearchesOngoing = mSearches.Length();

toolkit/components/autocomplete/nsIAutoCompleteInput.idl

@@ -178,4 +178,11 @@ interface nsIAutoCompleteInput : nsISupports
    * The userContextId of the current browser.
    */
   readonly attribute unsigned long userContextId;
+
+  /**
+   * Indicates whether the previous result should be invalidated due to dynamic
+   * list updates. If search content is updated, we shouldn't use the previous
+   * search result.
+   */
+  readonly attribute boolean invalidatePreviousResult;
 };