diff --git a/widget/ContentCache.cpp b/widget/ContentCache.cpp
index 9aac80145ff9..92cf4af6eda4 100644
--- a/widget/ContentCache.cpp
+++ b/widget/ContentCache.cpp
@@ -72,13 +72,11 @@ public:
  * mozilla::ContentCache
  *****************************************************************************/
 
-PRLogModuleInfo* sContentCacheLog = nullptr;
+LazyLogModule sContentCacheLog("ContentCacheWidgets");
 
 ContentCache::ContentCache()
+  : mCompositionStart(UINT32_MAX)
 {
-  if (!sContentCacheLog) {
-    sContentCacheLog = PR_NewLogModule("ContentCacheWidgets");
-  }
 }
 
 /*****************************************************************************
@@ -96,6 +94,7 @@ ContentCacheInChild::Clear()
   MOZ_LOG(sContentCacheLog, LogLevel::Info,
     ("ContentCacheInChild: 0x%p Clear()", this));
 
+  mCompositionStart = UINT32_MAX;
   mText.Truncate();
   mSelection.Clear();
   mFirstCharRect.SetEmpty();
@@ -286,6 +285,7 @@ ContentCacheInChild::CacheTextRects(nsIWidget* aWidget,
      this, aWidget, GetNotificationName(aNotification), mCaret.mOffset,
      GetBoolName(mCaret.IsValid())));
 
+  mCompositionStart = UINT32_MAX;
   mTextRectArray.Clear();
   mSelection.mAnchorCharRect.SetEmpty();
   mSelection.mFocusCharRect.SetEmpty();
@@ -300,12 +300,15 @@ ContentCacheInChild::CacheTextRects(nsIWidget* aWidget,
   RefPtr<TextComposition> textComposition =
     IMEStateManager::GetTextCompositionFor(aWidget);
   if (textComposition) {
+    // mCompositionStart may be updated by some composition event handlers.
+    // So, let's update it with the latest information.
+    mCompositionStart = textComposition->NativeOffsetOfStartComposition();
     // Note that TextComposition::String() may not be modified here because
     // it's modified after all edit action listeners are performed but this
     // is called while some of them are performed.
     uint32_t length = textComposition->LastData().Length();
     mTextRectArray.mRects.SetCapacity(length);
-    mTextRectArray.mStart = textComposition->NativeOffsetOfStartComposition();
+    mTextRectArray.mStart = mCompositionStart;
     uint32_t endOffset = mTextRectArray.mStart + length;
     for (uint32_t i = mTextRectArray.mStart; i < endOffset; i++) {
       LayoutDeviceIntRect charRect;
@@ -428,7 +431,6 @@ ContentCacheInChild::SetSelection(nsIWidget* aWidget,
 ContentCacheInParent::ContentCacheInParent()
   : ContentCache()
   , mCommitStringByRequest(nullptr)
-  , mCompositionStart(UINT32_MAX)
   , mPendingEventsNeedingAck(0)
   , mIsComposing(false)
 {
@@ -438,6 +440,7 @@ void
 ContentCacheInParent::AssignContent(const ContentCache& aOther,
                                     const IMENotification* aNotification)
 {
+  mCompositionStart = aOther.mCompositionStart;
   mText = aOther.mText;
   mSelection = aOther.mSelection;
   mFirstCharRect = aOther.mFirstCharRect;
@@ -450,7 +453,7 @@ ContentCacheInParent::AssignContent(const ContentCache& aOther,
      "Succeeded, mText.Length()=%u, mSelection={ mAnchor=%u, mFocus=%u, "
      "mWritingMode=%s, mAnchorCharRect=%s, mFocusCharRect=%s, mRect=%s }, "
      "mFirstCharRect=%s, mCaret={ mOffset=%u, mRect=%s }, mTextRectArray={ "
-     "mStart=%u, mRects.Length()=%u }, mEditorRect=%s",
+     "mStart=%u, mRects.Length()=%u }, mCompositionStart=%u, mEditorRect=%s",
      this, GetNotificationName(aNotification),
      mText.Length(), mSelection.mAnchor, mSelection.mFocus,
      GetWritingModeName(mSelection.mWritingMode).get(),
@@ -458,7 +461,8 @@ ContentCacheInParent::AssignContent(const ContentCache& aOther,
      GetRectText(mSelection.mFocusCharRect).get(),
      GetRectText(mSelection.mRect).get(), GetRectText(mFirstCharRect).get(),
      mCaret.mOffset, GetRectText(mCaret.mRect).get(), mTextRectArray.mStart,
-     mTextRectArray.mRects.Length(), GetRectText(mEditorRect).get()));
+     mTextRectArray.mRects.Length(), mCompositionStart,
+     GetRectText(mEditorRect).get()));
 }
 
 bool
@@ -835,6 +839,10 @@ ContentCacheInParent::OnCompositionEvent(const WidgetCompositionEvent& aEvent)
 
   mIsComposing = !aEvent.CausesDOMCompositionEndEvent();
 
+  if (!mIsComposing) {
+    mCompositionStart = UINT32_MAX;
+  }
+
   // During REQUEST_TO_COMMIT_COMPOSITION or REQUEST_TO_CANCEL_COMPOSITION,
   // widget usually sends a eCompositionChange and/or eCompositionCommit event
   // to finalize or clear the composition, respectively.  In this time,
diff --git a/widget/ContentCache.h b/widget/ContentCache.h
index dde6f051937b..73814f63baed 100644
--- a/widget/ContentCache.h
+++ b/widget/ContentCache.h
@@ -41,6 +41,9 @@ protected:
   // Whole text in the target
   nsString mText;
 
+  // Start offset of the composition string.
+  uint32_t mCompositionStart;
+
   struct Selection final
   {
     // Following values are offset in "flat text".
@@ -360,8 +363,6 @@ private:
   // composition.  Then, data value of dispatched composition events should
   // be stored into the instance.
   nsAString* mCommitStringByRequest;
-  // Start offset of the composition string.
-  uint32_t mCompositionStart;
   // mPendingEventsNeedingAck is increased before sending a composition event or
   // a selection event and decreased after they are received in the child
   // process.
diff --git a/widget/nsGUIEventIPC.h b/widget/nsGUIEventIPC.h
index 436eab6f9ace..b39744b583dd 100644
--- a/widget/nsGUIEventIPC.h
+++ b/widget/nsGUIEventIPC.h
@@ -975,6 +975,7 @@ struct ParamTraits<mozilla::ContentCache>
 
   static void Write(Message* aMsg, const paramType& aParam)
   {
+    WriteParam(aMsg, aParam.mCompositionStart);
     WriteParam(aMsg, aParam.mText);
     WriteParam(aMsg, aParam.mSelection.mAnchor);
     WriteParam(aMsg, aParam.mSelection.mFocus);
@@ -992,7 +993,8 @@ struct ParamTraits<mozilla::ContentCache>
 
   static bool Read(const Message* aMsg, PickleIterator* aIter, paramType* aResult)
   {
-    return ReadParam(aMsg, aIter, &aResult->mText) &&
+    return ReadParam(aMsg, aIter, &aResult->mCompositionStart) &&
+           ReadParam(aMsg, aIter, &aResult->mText) &&
            ReadParam(aMsg, aIter, &aResult->mSelection.mAnchor) &&
            ReadParam(aMsg, aIter, &aResult->mSelection.mFocus) &&
            ReadParam(aMsg, aIter, &aResult->mSelection.mWritingMode) &&