--- a/widget/windows/KeyboardLayout.cpp
+++ b/widget/windows/KeyboardLayout.cpp
@@ -3674,27 +3674,27 @@ KeyboardLayout::InitNativeKey(NativeKey&
 
     // At keydown message handling, we need to forget the first dead key
     // because there is no guarantee coming WM_KEYUP for the second dead
     // key before next WM_KEYDOWN.  E.g., due to auto key repeat or pressing
     // another dead key before releasing current key.  Therefore, we can
     // set only a character for current key for keyup event.
     if (mActiveDeadKey < 0) {
       aNativeKey.mCommittedCharsAndModifiers =
-        mVirtualKeys[virtualKeyIndex].GetUniChars(aModKeyState);
+        GetUniCharsAndModifiers(virtualKey, aModKeyState);
       return;
     }
 
-    int32_t activeDeadKeyIndex = GetKeyIndex(mActiveDeadKey);
-    if (activeDeadKeyIndex < 0 || activeDeadKeyIndex >= NS_NUM_OF_KEYS) {
+    if (NS_WARN_IF(!IsPrintableCharKey(mActiveDeadKey))) {
 #if defined(DEBUG) || defined(MOZ_CRASHREPORTER)
       nsPrintfCString warning("The virtual key index (%d) of mActiveDeadKey "
                               "(0x%02X) is not a printable key (virtualKey="
                               "0x%02X)",
-                              activeDeadKeyIndex, mActiveDeadKey, virtualKey);
+                              GetKeyIndex(mActiveDeadKey), mActiveDeadKey,
+                              virtualKey);
       NS_WARNING(warning.get());
 #ifdef MOZ_CRASHREPORTER
       CrashReporter::AppendAppNotesToCrashReport(
                        NS_LITERAL_CSTRING("\n") + warning);
 #endif // #ifdef MOZ_CRASHREPORTER
 #endif // #if defined(DEBUG) || defined(MOZ_CRASHREPORTER)
       MOZ_CRASH("Trying to reference out of range of mVirtualKeys");
     }
@@ -3703,48 +3703,47 @@ KeyboardLayout::InitNativeKey(NativeKey&
     // (e.g., "Russian - Mnemonic" keyboard layout's 's' -> 'c').
     if (MaybeInitNativeKeyWithCompositeChar(aNativeKey, aModKeyState)) {
       return;
     }
 
     // Otherwise, dead key followed by another dead key causes inputting both
     // character.
     UniCharsAndModifiers prevDeadChars =
-      mVirtualKeys[activeDeadKeyIndex].GetUniChars(mDeadKeyShiftState);
+      GetUniCharsAndModifiers(mActiveDeadKey, mDeadKeyShiftState);
     UniCharsAndModifiers newChars =
-      mVirtualKeys[virtualKeyIndex].GetUniChars(aModKeyState);
+      GetUniCharsAndModifiers(virtualKey, aModKeyState);
     // But keypress events should be fired for each committed character.
     aNativeKey.mCommittedCharsAndModifiers = prevDeadChars + newChars;
     if (isKeyDown) {
       DeactivateDeadKeyState();
     }
     return;
   }
 
   if (MaybeInitNativeKeyWithCompositeChar(aNativeKey, aModKeyState)) {
     return;
   }
 
   UniCharsAndModifiers baseChars =
-    mVirtualKeys[virtualKeyIndex].GetUniChars(aModKeyState);
+    GetUniCharsAndModifiers(virtualKey, aModKeyState);
   if (mActiveDeadKey < 0) {
     // No dead-keys are active. Just return the produced characters.
     aNativeKey.mCommittedCharsAndModifiers = baseChars;
     return;
   }
 
-  int32_t activeDeadKeyIndex = GetKeyIndex(mActiveDeadKey);
-  if (NS_WARN_IF(activeDeadKeyIndex < 0)) {
+  if (NS_WARN_IF(!IsPrintableCharKey(mActiveDeadKey))) {
     return;
   }
 
   // There is no valid dead-key and base character combination.
   // Return dead-key character followed by base character.
   UniCharsAndModifiers deadChars =
-    mVirtualKeys[activeDeadKeyIndex].GetUniChars(mDeadKeyShiftState);
+    GetUniCharsAndModifiers(mActiveDeadKey, mDeadKeyShiftState);
   // But keypress events should be fired for each committed character.
   aNativeKey.mCommittedCharsAndModifiers = deadChars + baseChars;
   if (isKeyDown) {
     DeactivateDeadKeyState();
   }
 }
 
 bool
@@ -3756,23 +3755,22 @@ KeyboardLayout::MaybeInitNativeKeyWithCo
     return false;
   }
 
   int32_t activeDeadKeyIndex = GetKeyIndex(mActiveDeadKey);
   if (NS_WARN_IF(activeDeadKeyIndex < 0)) {
     return false;
   }
 
-  int32_t virtualKeyIndex = GetKeyIndex(aNativeKey.mOriginalVirtualKeyCode);
-  if (NS_WARN_IF(virtualKeyIndex < 0)) {
+  if (NS_WARN_IF(!IsPrintableCharKey(aNativeKey.mOriginalVirtualKeyCode))) {
     return false;
   }
 
   UniCharsAndModifiers baseChars =
-    mVirtualKeys[virtualKeyIndex].GetUniChars(aModKeyState);
+    GetUniCharsAndModifiers(aNativeKey.mOriginalVirtualKeyCode, aModKeyState);
   if (baseChars.IsEmpty() || !baseChars.mChars[0]) {
     return false;
   }
 
   char16_t compositeChar =
     mVirtualKeys[activeDeadKeyIndex].GetCompositeChar(mDeadKeyShiftState,
                                                       baseChars.mChars[0]);
   if (!compositeChar) {
@@ -3787,25 +3785,24 @@ KeyboardLayout::MaybeInitNativeKeyWithCo
     DeactivateDeadKeyState();
   }
   return true;
 }
 
 UniCharsAndModifiers
 KeyboardLayout::GetUniCharsAndModifiers(
                   uint8_t aVirtualKey,
-                  const ModifierKeyState& aModKeyState) const
+                  VirtualKey::ShiftState aShiftState) const
 {
   UniCharsAndModifiers result;
   int32_t key = GetKeyIndex(aVirtualKey);
   if (key < 0) {
     return result;
   }
-  return mVirtualKeys[key].
-    GetUniChars(VirtualKey::ModifiersToShiftState(aModKeyState.GetModifiers()));
+  return mVirtualKeys[key].GetUniChars(aShiftState);
 }
 
 void
 KeyboardLayout::LoadLayout(HKL aLayout)
 {
   mIsPendingToRestoreKeyboardLayout = false;
 
   if (mKeyboardLayout == aLayout) {

--- a/widget/windows/KeyboardLayout.h
+++ b/widget/windows/KeyboardLayout.h
@@ -656,16 +656,23 @@ private:
    * InitNativeKey() must be called when actually widget receives WM_KEYDOWN or
    * WM_KEYUP.  This method is stateful.  This saves current dead key state at
    * WM_KEYDOWN.  Additionally, computes current inputted character(s) and set
    * them to the aNativeKey.
    */
   void InitNativeKey(NativeKey& aNativeKey,
                      const ModifierKeyState& aModKeyState);
 
+  /**
+   * See the comment of GetUniCharsAndModifiers() below.
+   */
+  UniCharsAndModifiers GetUniCharsAndModifiers(
+                         uint8_t aVirtualKey,
+                         VirtualKey::ShiftState aShiftState) const;
+
 public:
   static KeyboardLayout* GetInstance();
   static void Shutdown();
   static void NotifyIdleServiceOfUserActivity();
 
   static bool IsPrintableCharKey(uint8_t aVirtualKey);
 
   /**
@@ -689,20 +696,27 @@ public:
    */
   bool MaybeInitNativeKeyWithCompositeChar(
          NativeKey& aNativeKey,
          const ModifierKeyState& aModKeyState);
 
   /**
    * GetUniCharsAndModifiers() returns characters which is inputted by the
    * aVirtualKey with aModKeyState.  This method isn't stateful.
+   * Note that if the combination causes text input, the result's Ctrl and
+   * Alt key state are never active.
    */
   UniCharsAndModifiers GetUniCharsAndModifiers(
                          uint8_t aVirtualKey,
-                         const ModifierKeyState& aModKeyState) const;
+                         const ModifierKeyState& aModKeyState) const
+  {
+    VirtualKey::ShiftState shiftState =
+      VirtualKey::ModifierKeyStateToShiftState(aModKeyState);
+    return GetUniCharsAndModifiers(aVirtualKey, shiftState);
+  }
 
   /**
    * OnLayoutChange() must be called before the first keydown message is
    * received.  LoadLayout() changes the keyboard state, that causes breaking
    * dead key state.  Therefore, we need to load the layout before the first
    * keydown message.
    */
   void OnLayoutChange(HKL aKeyboardLayout)