--- a/widget/windows/KeyboardLayout.cpp
+++ b/widget/windows/KeyboardLayout.cpp
@@ -620,26 +620,26 @@ ToString(const MSG& aMSG)
 static const nsCString
 ToString(const UniCharsAndModifiers& aUniCharsAndModifiers)
 {
   if (aUniCharsAndModifiers.IsEmpty()) {
     return NS_LITERAL_CSTRING("{}");
   }
   nsAutoCString result;
   result.AssignLiteral("{ ");
-  result.Append(GetCharacterCodeName(aUniCharsAndModifiers.mChars[0]));
+  result.Append(GetCharacterCodeName(aUniCharsAndModifiers.CharAt(0)));
   for (uint32_t i = 1; i < aUniCharsAndModifiers.mLength; ++i) {
     if (aUniCharsAndModifiers.mModifiers[i - 1] !=
           aUniCharsAndModifiers.mModifiers[i]) {
       result.AppendLiteral(" [");
       result.Append(GetModifiersName(aUniCharsAndModifiers.mModifiers[0]));
       result.AppendLiteral("]");
     }
     result.AppendLiteral(", ");
-    result.Append(GetCharacterCodeName(aUniCharsAndModifiers.mChars[i]));
+    result.Append(GetCharacterCodeName(aUniCharsAndModifiers.CharAt(i)));
   }
   result.AppendLiteral(" [");
   uint32_t lastIndex = aUniCharsAndModifiers.mLength - 1;
   result.Append(GetModifiersName(aUniCharsAndModifiers.mModifiers[lastIndex]));
   result.AppendLiteral("] }");
   return result;
 }
 
@@ -3142,30 +3142,30 @@ NativeKey::ComputeInputtingStringWithKey
                                           &mUnshiftedLatinChar,
                                           &mShiftedLatinChar);
 
   // If the mShiftedLatinChar isn't 0, the key code is NS_VK_[A-Z].
   if (mShiftedLatinChar) {
     // If the produced characters of the key on current keyboard layout
     // are same as computed Latin characters, we shouldn't append the
     // Latin characters to alternativeCharCode.
-    if (mUnshiftedLatinChar == mUnshiftedString.mChars[0] &&
-        mShiftedLatinChar == mShiftedString.mChars[0]) {
+    if (mUnshiftedLatinChar == mUnshiftedString.CharAt(0) &&
+        mShiftedLatinChar == mShiftedString.CharAt(0)) {
       mShiftedLatinChar = mUnshiftedLatinChar = 0;
     }
   } else if (mUnshiftedLatinChar) {
     // If the mShiftedLatinChar is 0, the mKeyCode doesn't produce
     // alphabet character.  At that time, the character may be produced
     // with Shift key.  E.g., on French keyboard layout, NS_VK_PERCENT
     // key produces LATIN SMALL LETTER U WITH GRAVE (U+00F9) without
     // Shift key but with Shift key, it produces '%'.
     // If the mUnshiftedLatinChar is produced by the key on current
     // keyboard layout, we shouldn't append it to alternativeCharCode.
-    if (mUnshiftedLatinChar == mUnshiftedString.mChars[0] ||
-        mUnshiftedLatinChar == mShiftedString.mChars[0]) {
+    if (mUnshiftedLatinChar == mUnshiftedString.CharAt(0) ||
+        mUnshiftedLatinChar == mShiftedString.CharAt(0)) {
       mUnshiftedLatinChar = 0;
     }
   }
 
   if (!mModKeyState.IsControl()) {
     return;
   }
 
@@ -3373,17 +3373,17 @@ NativeKey::WillDispatchKeyboardEvent(Wid
         mInputtingStringAndModifiers.mModifiers[aIndex - skipUniChars]);
       modKeyState.InitInputEvent(aKeyboardEvent);
       MOZ_LOG(sNativeKeyLogger, LogLevel::Info,
         ("%p   NativeKey::WillDispatchKeyboardEvent(), "
          "setting %uth modifier state to %s",
          this, aIndex + 1, ToString(modKeyState).get()));
     }
     uint16_t uniChar =
-      mInputtingStringAndModifiers.mChars[aIndex - skipUniChars];
+      mInputtingStringAndModifiers.CharAt(aIndex - skipUniChars);
 
     // The mCharCode was set from mKeyValue. However, for example, when Ctrl key
     // is pressed, its value should indicate an ASCII character for backward
     // compatibility rather than inputting character without the modifiers.
     // Therefore, we need to modify mCharCode value here.
     aKeyboardEvent.SetCharCode(uniChar);
     MOZ_LOG(sNativeKeyLogger, LogLevel::Info,
       ("%p   NativeKey::WillDispatchKeyboardEvent(), "
@@ -3409,20 +3409,20 @@ NativeKey::WillDispatchKeyboardEvent(Wid
       count = longestLength - aIndex;
     }
   } else {
     count = longestLength;
   }
   for (size_t i = 0; i < count; ++i) {
     uint16_t shiftedChar = 0, unshiftedChar = 0;
     if (skipShiftedChars <= aIndex + i) {
-      shiftedChar = mShiftedString.mChars[aIndex + i - skipShiftedChars];
+      shiftedChar = mShiftedString.CharAt(aIndex + i - skipShiftedChars);
     }
     if (skipUnshiftedChars <= aIndex + i) {
-      unshiftedChar = mUnshiftedString.mChars[aIndex + i - skipUnshiftedChars];
+      unshiftedChar = mUnshiftedString.CharAt(aIndex + i - skipUnshiftedChars);
     }
 
     if (shiftedChar || unshiftedChar) {
       AlternativeCharCode chars(unshiftedChar, shiftedChar);
       altArray.AppendElement(chars);
     }
 
     if (!isLastIndex) {
@@ -3443,18 +3443,18 @@ NativeKey::WillDispatchKeyboardEvent(Wid
     char16_t charForOEMKeyCode = 0;
     switch (mVirtualKeyCode) {
       case VK_OEM_PLUS:   charForOEMKeyCode = '+'; break;
       case VK_OEM_COMMA:  charForOEMKeyCode = ','; break;
       case VK_OEM_MINUS:  charForOEMKeyCode = '-'; break;
       case VK_OEM_PERIOD: charForOEMKeyCode = '.'; break;
     }
     if (charForOEMKeyCode &&
-        charForOEMKeyCode != mUnshiftedString.mChars[0] &&
-        charForOEMKeyCode != mShiftedString.mChars[0] &&
+        charForOEMKeyCode != mUnshiftedString.CharAt(0) &&
+        charForOEMKeyCode != mShiftedString.CharAt(0) &&
         charForOEMKeyCode != mUnshiftedLatinChar &&
         charForOEMKeyCode != mShiftedLatinChar) {
       AlternativeCharCode OEMChars(charForOEMKeyCode, charForOEMKeyCode);
       altArray.AppendElement(OEMChars);
     }
   }
 }
 
@@ -3780,22 +3780,22 @@ KeyboardLayout::MaybeInitNativeKeyWithCo
 
   if (NS_WARN_IF(!IsPrintableCharKey(mActiveDeadKey)) ||
       NS_WARN_IF(!IsPrintableCharKey(aNativeKey.mOriginalVirtualKeyCode))) {
     return false;
   }
 
   UniCharsAndModifiers baseChars =
     GetUniCharsAndModifiers(aNativeKey.mOriginalVirtualKeyCode, aModKeyState);
-  if (baseChars.IsEmpty() || !baseChars.mChars[0]) {
+  if (baseChars.IsEmpty() || !baseChars.CharAt(0)) {
     return false;
   }
 
   char16_t compositeChar =
-    GetCompositeChar(mActiveDeadKey, mDeadKeyShiftState, baseChars.mChars[0]);
+    GetCompositeChar(mActiveDeadKey, mDeadKeyShiftState, baseChars.CharAt(0));
   if (!compositeChar) {
     return false;
   }
 
   // Active dead-key and base character does produce exactly one composite
   // character.
   aNativeKey.mCommittedCharsAndModifiers.Append(compositeChar,
                                                 baseChars.mModifiers[0]);
@@ -4429,25 +4429,25 @@ KeyboardLayout::ConvertNativeKeyCodeToDO
     case VK_ABNT_C1:
     {
       NS_ASSERTION(IsPrintableCharKey(aNativeKeyCode),
                    "The key must be printable");
       ModifierKeyState modKeyState(0);
       UniCharsAndModifiers uniChars =
         GetUniCharsAndModifiers(aNativeKeyCode, modKeyState);
       if (uniChars.mLength != 1 ||
-          uniChars.mChars[0] < ' ' || uniChars.mChars[0] > 0x7F) {
+          uniChars.CharAt(0) < ' ' || uniChars.CharAt(0) > 0x7F) {
         modKeyState.Set(MODIFIER_SHIFT);
         uniChars = GetUniCharsAndModifiers(aNativeKeyCode, modKeyState);
         if (uniChars.mLength != 1 ||
-            uniChars.mChars[0] < ' ' || uniChars.mChars[0] > 0x7F) {
+            uniChars.CharAt(0) < ' ' || uniChars.CharAt(0) > 0x7F) {
           return 0;
         }
       }
-      return WidgetUtils::ComputeKeyCodeFromChar(uniChars.mChars[0]);
+      return WidgetUtils::ComputeKeyCodeFromChar(uniChars.CharAt(0));
     }
 
     // IE sets 0xC2 to the DOM keyCode for VK_ABNT_C2.  However, we're already
     // using NS_VK_SEPARATOR for the separator key on Mac and Linux.  Therefore,
     // We should keep consistency between Gecko on all platforms rather than
     // with other browsers since a lot of keyCode values are already different
     // between browsers.
     case VK_ABNT_C2:

--- a/widget/windows/KeyboardLayout.h
+++ b/widget/windows/KeyboardLayout.h
@@ -52,41 +52,49 @@ static const uint32_t sModifierKeyMap[][
   { nsIWidget::CTRL_L,    VK_CONTROL, VK_LCONTROL },
   { nsIWidget::CTRL_R,    VK_CONTROL, VK_RCONTROL },
   { nsIWidget::ALT_L,     VK_MENU,    VK_LMENU },
   { nsIWidget::ALT_R,     VK_MENU,    VK_RMENU }
 };
 
 class KeyboardLayout;
 
-struct UniCharsAndModifiers
+class UniCharsAndModifiers final
 {
+public:
   // Dead-key + up to 4 characters
-  char16_t mChars[5];
   Modifiers mModifiers[5];
   uint32_t  mLength;
 
   UniCharsAndModifiers() : mLength(0) {}
   UniCharsAndModifiers operator+(const UniCharsAndModifiers& aOther) const;
   UniCharsAndModifiers& operator+=(const UniCharsAndModifiers& aOther);
 
   /**
    * Append a pair of unicode character and the final modifier.
    */
   void Append(char16_t aUniChar, Modifiers aModifiers);
   void Clear() { mLength = 0; }
   bool IsEmpty() const { return !mLength; }
 
+  char16_t CharAt(size_t aIndex) const
+  {
+    MOZ_ASSERT(aIndex < mLength);
+    return mChars[aIndex];
+  }
   void FillModifiers(Modifiers aModifiers);
 
   bool UniCharsEqual(const UniCharsAndModifiers& aOther) const;
   bool UniCharsCaseInsensitiveEqual(const UniCharsAndModifiers& aOther) const;
   bool BeginsWith(const UniCharsAndModifiers& aOther) const;
 
   nsString ToString() const { return nsString(mChars, mLength); }
+
+private:
+  char16_t mChars[5];
 };
 
 struct DeadKeyEntry;
 class DeadKeyTable;
 
 
 class VirtualKey
 {