--- a/widget/ContentCache.cpp
+++ b/widget/ContentCache.cpp
@@ -102,17 +102,17 @@ LazyLogModule sContentCacheLog("ContentC
 void ContentCacheInChild::Clear() {
   MOZ_LOG(sContentCacheLog, LogLevel::Info, ("0x%p Clear()", this));
 
   mCompositionStart.reset();
   mLastCommit.reset();
   mText.Truncate();
   mSelection.reset();
   mFirstCharRect.SetEmpty();
-  mCaret.Clear();
+  mCaret.reset();
   mTextRectArray.Clear();
   mLastCommitStringTextRectArray.Clear();
   mEditorRect.SetEmpty();
 }
 
 void ContentCacheInChild::OnCompositionEvent(
     const WidgetCompositionEvent& aCompositionEvent) {
   if (aCompositionEvent.CausesDOMCompositionEndEvent()) {
@@ -167,17 +167,17 @@ bool ContentCacheInChild::CacheAll(nsIWi
 }
 
 bool ContentCacheInChild::CacheSelection(nsIWidget* aWidget,
                                          const IMENotification* aNotification) {
   MOZ_LOG(sContentCacheLog, LogLevel::Info,
           ("0x%p CacheSelection(aWidget=0x%p, aNotification=%s)", this, aWidget,
            GetNotificationName(aNotification)));
 
-  mCaret.Clear();
+  mCaret.reset();
   mSelection.reset();
 
   nsEventStatus status = nsEventStatus_eIgnore;
   WidgetQueryContentEvent selection(true, eQuerySelectedText, aWidget);
   aWidget->DispatchEvent(&selection, status);
   if (NS_WARN_IF(!selection.mSucceeded)) {
     MOZ_LOG(sContentCacheLog, LogLevel::Error,
             ("0x%p CacheSelection(), FAILED, "
@@ -201,43 +201,41 @@ bool ContentCacheInChild::CacheSelection
 }
 
 bool ContentCacheInChild::CacheCaret(nsIWidget* aWidget,
                                      const IMENotification* aNotification) {
   MOZ_LOG(sContentCacheLog, LogLevel::Info,
           ("0x%p CacheCaret(aWidget=0x%p, aNotification=%s)", this, aWidget,
            GetNotificationName(aNotification)));
 
-  mCaret.Clear();
+  mCaret.reset();
 
   if (NS_WARN_IF(mSelection.isNothing())) {
     return false;
   }
 
   // XXX Should be mSelection.mFocus?
-  mCaret.mOffset = mSelection->StartOffset();
+  uint32_t offset = mSelection->StartOffset();
 
   nsEventStatus status = nsEventStatus_eIgnore;
   WidgetQueryContentEvent caretRect(true, eQueryCaretRect, aWidget);
-  caretRect.InitForQueryCaretRect(mCaret.mOffset);
+  caretRect.InitForQueryCaretRect(offset);
   aWidget->DispatchEvent(&caretRect, status);
   if (NS_WARN_IF(!caretRect.mSucceeded)) {
     MOZ_LOG(sContentCacheLog, LogLevel::Error,
             ("0x%p CacheCaret(), FAILED, "
              "couldn't retrieve the caret rect at offset=%u",
-             this, mCaret.mOffset));
-    mCaret.Clear();
+             this, offset));
     return false;
   }
-  mCaret.mRect = caretRect.mReply.mRect;
+  mCaret.emplace(offset, caretRect.mReply.mRect);
   MOZ_LOG(sContentCacheLog, LogLevel::Info,
           ("0x%p CacheCaret(), Succeeded, "
-           "mSelection=%s, mCaret={ mOffset=%u, mRect=%s }",
-           this, ToString(mSelection).c_str(), mCaret.mOffset,
-           GetRectText(mCaret.mRect).get()));
+           "mSelection=%s, mCaret=%s",
+           this, ToString(mSelection).c_str(), ToString(mCaret).c_str()));
   return true;
 }
 
 bool ContentCacheInChild::CacheEditorRect(
     nsIWidget* aWidget, const IMENotification* aNotification) {
   MOZ_LOG(sContentCacheLog, LogLevel::Info,
           ("0x%p CacheEditorRect(aWidget=0x%p, aNotification=%s)", this,
            aWidget, GetNotificationName(aNotification)));
@@ -336,21 +334,20 @@ bool ContentCacheInChild::QueryCharRectA
     return false;
   }
   aCharRectArray = std::move(textRects.mReply.mRectArray);
   return true;
 }
 
 bool ContentCacheInChild::CacheTextRects(nsIWidget* aWidget,
                                          const IMENotification* aNotification) {
-  MOZ_LOG(sContentCacheLog, LogLevel::Info,
-          ("0x%p CacheTextRects(aWidget=0x%p, aNotification=%s), "
-           "mCaret={ mOffset=%u, IsValid()=%s }",
-           this, aWidget, GetNotificationName(aNotification), mCaret.mOffset,
-           GetBoolName(mCaret.IsValid())));
+  MOZ_LOG(
+      sContentCacheLog, LogLevel::Info,
+      ("0x%p CacheTextRects(aWidget=0x%p, aNotification=%s), mCaret=%s", this,
+       aWidget, GetNotificationName(aNotification), ToString(mCaret).c_str()));
 
   mCompositionStart.reset();
   mTextRectArray.Clear();
   mLastCommitStringTextRectArray.Clear();
   mFirstCharRect.SetEmpty();
 
   if (NS_WARN_IF(mSelection.isNothing())) {
     return false;
@@ -605,33 +602,31 @@ void ContentCacheInParent::AssignContent
       }
     } else if (mCompositionStart.isSome() && mSelection.isSome() &&
                mCompositionStart.value() != mSelection->StartOffset()) {
       mCompositionStart = Some(mSelection->StartOffset());
       mPendingCommitLength = 0;
     }
   }
 
-  MOZ_LOG(
-      sContentCacheLog, LogLevel::Info,
-      ("0x%p AssignContent(aNotification=%s), "
-       "Succeeded, mText.Length()=%u, mSelection=%s, mFirstCharRect=%s, "
-       "mCaret={ mOffset=%u, mRect=%s }, mTextRectArray={ mStart=%u, "
-       "mRects.Length()=%zu }, mWidgetHasComposition=%s, "
-       "mPendingCompositionCount=%u, mCompositionStart=%u, "
-       "mPendingCommitLength=%u, mEditorRect=%s, "
-       "mLastCommitStringTextRectArray={ mStart=%u, mRects.Length()=%zu }",
-       this, GetNotificationName(aNotification), mText.Length(),
-       ToString(mSelection).c_str(), GetRectText(mFirstCharRect).get(),
-       mCaret.mOffset, GetRectText(mCaret.mRect).get(), mTextRectArray.mStart,
-       mTextRectArray.mRects.Length(), GetBoolName(mWidgetHasComposition),
-       mPendingCompositionCount, mCompositionStart.valueOr(UINT32_MAX),
-       mPendingCommitLength, GetRectText(mEditorRect).get(),
-       mLastCommitStringTextRectArray.mStart,
-       mLastCommitStringTextRectArray.mRects.Length()));
+  MOZ_LOG(sContentCacheLog, LogLevel::Info,
+          ("0x%p AssignContent(aNotification=%s), "
+           "Succeeded, mText.Length()=%u, mSelection=%s, mFirstCharRect=%s, "
+           "mCaret=%s, mTextRectArray={ mStart=%u, mRects.Length()=%zu }, "
+           "mWidgetHasComposition=%s, mPendingCompositionCount=%u, "
+           "mCompositionStart=%u, mPendingCommitLength=%u, mEditorRect=%s, "
+           "mLastCommitStringTextRectArray={ mStart=%u, mRects.Length()=%zu }",
+           this, GetNotificationName(aNotification), mText.Length(),
+           ToString(mSelection).c_str(), GetRectText(mFirstCharRect).get(),
+           ToString(mCaret).c_str(), mTextRectArray.mStart,
+           mTextRectArray.mRects.Length(), GetBoolName(mWidgetHasComposition),
+           mPendingCompositionCount, mCompositionStart.valueOr(UINT32_MAX),
+           mPendingCommitLength, GetRectText(mEditorRect).get(),
+           mLastCommitStringTextRectArray.mStart,
+           mLastCommitStringTextRectArray.mRects.Length()));
 }
 
 bool ContentCacheInParent::HandleQueryContentEvent(
     WidgetQueryContentEvent& aEvent, nsIWidget* aWidget) const {
   MOZ_ASSERT(aWidget);
 
   aEvent.mSucceeded = false;
   aEvent.mReply.mFocusedWidget = aWidget;
@@ -1128,27 +1123,26 @@ bool ContentCacheInParent::GetUnionTextR
 
   return !aUnionTextRect.IsEmpty();
 }
 
 bool ContentCacheInParent::GetCaretRect(uint32_t aOffset,
                                         bool aRoundToExistingOffset,
                                         LayoutDeviceIntRect& aCaretRect) const {
   MOZ_LOG(sContentCacheLog, LogLevel::Info,
-          ("0x%p GetCaretRect(aOffset=%u, "
-           "aRoundToExistingOffset=%s), "
-           "mCaret={ mOffset=%u, mRect=%s, IsValid()=%s }, mTextRectArray={ "
-           "mStart=%u, mRects.Length()=%zu }, mSelection=%s, mFirstCharRect=%s",
-           this, aOffset, GetBoolName(aRoundToExistingOffset), mCaret.mOffset,
-           GetRectText(mCaret.mRect).get(), GetBoolName(mCaret.IsValid()),
-           mTextRectArray.mStart, mTextRectArray.mRects.Length(),
-           ToString(mSelection).c_str(), GetRectText(mFirstCharRect).get()));
+          ("0x%p GetCaretRect(aOffset=%u, aRoundToExistingOffset=%s), "
+           "mCaret=%s, mTextRectArray={ mStart=%u, mRects.Length()=%zu }, "
+           "mSelection=%s, mFirstCharRect=%s",
+           this, aOffset, GetBoolName(aRoundToExistingOffset),
+           ToString(mCaret).c_str(), mTextRectArray.mStart,
+           mTextRectArray.mRects.Length(), ToString(mSelection).c_str(),
+           GetRectText(mFirstCharRect).get()));
 
-  if (mCaret.IsValid() && mCaret.mOffset == aOffset) {
-    aCaretRect = mCaret.mRect;
+  if (mCaret.isSome() && mCaret->mOffset == aOffset) {
+    aCaretRect = mCaret->mRect;
     return true;
   }
 
   // Guess caret rect from the text rect if it's stored.
   if (!GetTextRect(aOffset, aRoundToExistingOffset, aCaretRect)) {
     // There might be previous character rect in the cache.  If so, we can
     // guess the caret rect with it.
     if (!aOffset ||
@@ -1164,19 +1158,19 @@ bool ContentCacheInParent::GetCaretRect(
       aCaretRect.MoveToX(aCaretRect.XMost());
     }
   }
 
   // XXX This is not bidi aware because we don't cache each character's
   //     direction.  However, this is usually used by IME, so, assuming the
   //     character is in LRT context must not cause any problem.
   if (mSelection.isSome() && mSelection->mWritingMode.IsVertical()) {
-    aCaretRect.SetHeight(mCaret.IsValid() ? mCaret.mRect.Height() : 1);
+    aCaretRect.SetHeight(mCaret.isSome() ? mCaret->mRect.Height() : 1);
   } else {
-    aCaretRect.SetWidth(mCaret.IsValid() ? mCaret.mRect.Width() : 1);
+    aCaretRect.SetWidth(mCaret.isSome() ? mCaret->mRect.Width() : 1);
   }
   return true;
 }
 
 bool ContentCacheInParent::OnCompositionEvent(
     const WidgetCompositionEvent& aEvent) {
   MOZ_LOG(
       sContentCacheLog, LogLevel::Info,

--- a/widget/ContentCache.h
+++ b/widget/ContentCache.h
@@ -159,30 +159,38 @@ class ContentCache {
   // Stores first char rect because Yosemite's Japanese IME sometimes tries
   // to query it.  If there is no text, this is caret rect.
   LayoutDeviceIntRect mFirstCharRect;
 
   struct Caret final {
     uint32_t mOffset;
     LayoutDeviceIntRect mRect;
 
-    Caret() : mOffset(UINT32_MAX) {}
+    explicit Caret(uint32_t aOffset, LayoutDeviceIntRect aCaretRect)
+        : mOffset(aOffset), mRect(aCaretRect) {}
+
+    uint32_t Offset() const { return mOffset; }
+    bool HasRect() const { return !mRect.IsEmpty(); }
 
-    void Clear() {
-      mOffset = UINT32_MAX;
-      mRect.SetEmpty();
+    friend std::ostream& operator<<(std::ostream& aStream,
+                                    const Caret& aCaret) {
+      aStream << "{ mOffset=" << aCaret.mOffset;
+      if (aCaret.HasRect()) {
+        aStream << ", mRect=" << aCaret.mRect;
+      }
+      return aStream << " }";
     }
 
-    bool IsValid() const { return mOffset != UINT32_MAX; }
+   private:
+    Caret() = default;
 
-    uint32_t Offset() const {
-      NS_ASSERTION(IsValid(), "The caller should check if the caret is valid");
-      return mOffset;
-    }
-  } mCaret;
+    friend struct IPC::ParamTraits<ContentCache::Caret>;
+    friend struct IPC::ParamTraits<Maybe<ContentCache::Caret>>;
+  };
+  Maybe<Caret> mCaret;
 
   struct TextRectArray final {
     uint32_t mStart;
     RectArray mRects;
 
     TextRectArray() : mStart(UINT32_MAX) {}
 
     void Clear() {
@@ -238,16 +246,17 @@ class ContentCache {
   TextRectArray mTextRectArray;
   TextRectArray mLastCommitStringTextRectArray;
 
   LayoutDeviceIntRect mEditorRect;
 
   friend class ContentCacheInParent;
   friend struct IPC::ParamTraits<ContentCache>;
   friend struct IPC::ParamTraits<ContentCache::Selection>;
+  friend struct IPC::ParamTraits<ContentCache::Caret>;
   friend std::ostream& operator<<(
       std::ostream& aStream,
       const Selection& aSelection);  // For e(Prev|Next)CharRect
 };
 
 class ContentCacheInChild final : public ContentCache {
  public:
   ContentCacheInChild() = default;

--- a/widget/nsGUIEventIPC.h
+++ b/widget/nsGUIEventIPC.h
@@ -1055,41 +1055,55 @@ struct ParamTraits<mozilla::ContentCache
            ReadParam(aMsg, aIter, &aResult->mAnchorCharRects[1]) &&
            ReadParam(aMsg, aIter, &aResult->mFocusCharRects[0]) &&
            ReadParam(aMsg, aIter, &aResult->mFocusCharRects[1]) &&
            ReadParam(aMsg, aIter, &aResult->mRect);
   }
 };
 
 template <>
+struct ParamTraits<mozilla::ContentCache::Caret> {
+  typedef mozilla::ContentCache::Caret paramType;
+
+  static void Write(Message* aMsg, const paramType& aParam) {
+    WriteParam(aMsg, aParam.mOffset);
+    WriteParam(aMsg, aParam.mRect);
+  }
+
+  static bool Read(const Message* aMsg, PickleIterator* aIter,
+                   paramType* aResult) {
+    return ReadParam(aMsg, aIter, &aResult->mOffset) &&
+           ReadParam(aMsg, aIter, &aResult->mRect);
+  }
+};
+
+template <>
 struct ParamTraits<mozilla::ContentCache> {
   typedef mozilla::ContentCache paramType;
 
   static void Write(Message* aMsg, const paramType& aParam) {
     WriteParam(aMsg, aParam.mCompositionStart);
     WriteParam(aMsg, aParam.mText);
     WriteParam(aMsg, aParam.mSelection);
     WriteParam(aMsg, aParam.mFirstCharRect);
-    WriteParam(aMsg, aParam.mCaret.mOffset);
-    WriteParam(aMsg, aParam.mCaret.mRect);
+    WriteParam(aMsg, aParam.mCaret);
     WriteParam(aMsg, aParam.mTextRectArray.mStart);
     WriteParam(aMsg, aParam.mTextRectArray.mRects);
     WriteParam(aMsg, aParam.mLastCommitStringTextRectArray.mStart);
     WriteParam(aMsg, aParam.mLastCommitStringTextRectArray.mRects);
     WriteParam(aMsg, aParam.mEditorRect);
   }
 
   static bool Read(const Message* aMsg, PickleIterator* aIter,
                    paramType* aResult) {
     return ReadParam(aMsg, aIter, &aResult->mCompositionStart) &&
            ReadParam(aMsg, aIter, &aResult->mText) &&
            ReadParam(aMsg, aIter, &aResult->mSelection) &&
            ReadParam(aMsg, aIter, &aResult->mFirstCharRect) &&
-           ReadParam(aMsg, aIter, &aResult->mCaret.mOffset) &&
-           ReadParam(aMsg, aIter, &aResult->mCaret.mRect) &&
+           ReadParam(aMsg, aIter, &aResult->mCaret) &&
            ReadParam(aMsg, aIter, &aResult->mTextRectArray.mStart) &&
            ReadParam(aMsg, aIter, &aResult->mTextRectArray.mRects) &&
            ReadParam(aMsg, aIter,
                      &aResult->mLastCommitStringTextRectArray.mStart) &&
            ReadParam(aMsg, aIter,
                      &aResult->mLastCommitStringTextRectArray.mRects) &&
            ReadParam(aMsg, aIter, &aResult->mEditorRect);
   }