--- a/layout/generic/nsBlockFrame.cpp
+++ b/layout/generic/nsBlockFrame.cpp
@@ -1391,20 +1391,26 @@ nsBlockFrame::Reflow(nsPresContext*     
                              containingBlockSize.GetPhysicalSize(parentWM));
       AbsPosReflowFlags flags = AbsPosReflowFlags::eConstrainHeight;
       if (cbWidthChanged) {
         flags |= AbsPosReflowFlags::eCBWidthChanged;
       }
       if (cbHeightChanged) {
         flags |= AbsPosReflowFlags::eCBHeightChanged;
       }
+      // Setup the line cursor here to optimize line searching for
+      // calculating hypothetical position of absolutely-positioned
+      // frames. The line cursor is immediately cleared afterward to
+      // avoid affecting the display list generation.
+      SetupLineCursor();
       absoluteContainer->Reflow(this, aPresContext, *reflowState,
                                 state.mReflowStatus,
                                 containingBlock, flags,
                                 &aMetrics.mOverflowAreas);
+      ClearLineCursor();
     }
   }
 
   FinishAndStoreOverflow(&aMetrics);
 
   // Clear the float manager pointer in the block reflow state so we
   // don't waste time translating the coordinate system back on a dead
   // float manager.
@@ -5486,65 +5492,69 @@ nsBlockInFlowLineIterator::nsBlockInFlow
   : mFrame(aFrame), mLineList(&aFrame->mLines)
 {
   *aFoundValidLine = false;
 
   nsIFrame* child = FindChildContaining(aFrame, aFindFrame);
   if (!child)
     return;
 
+  line_iterator line_end = aFrame->end_lines();
   // Try to use the cursor if it exists, otherwise fall back to the first line
-  nsLineBox* cursor = aFrame->GetLineCursor();
-  if (!cursor) {
-    line_iterator iter = aFrame->begin_lines();
-    if (iter != aFrame->end_lines()) {
-      cursor = iter;
-    }
-  }
-
-  if (cursor) {
+  if (nsLineBox* const cursor = aFrame->GetLineCursor()) {
+    mLine = line_end;
     // Perform a simultaneous forward and reverse search starting from the
     // line cursor.
     nsBlockFrame::line_iterator line = aFrame->line(cursor);
     nsBlockFrame::reverse_line_iterator rline = aFrame->rline(cursor);
-    nsBlockFrame::line_iterator line_end = aFrame->end_lines();
     nsBlockFrame::reverse_line_iterator rline_end = aFrame->rend_lines();
     // rline is positioned on the line containing 'cursor', so it's not
     // rline_end. So we can safely increment it (i.e. move it to one line
     // earlier) to start searching there.
     ++rline;
     while (line != line_end || rline != rline_end) {
       if (line != line_end) {
         if (line->Contains(child)) {
-          *aFoundValidLine = true;
           mLine = line;
-          return;
+          break;
         }
         ++line;
       }
       if (rline != rline_end) {
         if (rline->Contains(child)) {
-          *aFoundValidLine = true;
           mLine = rline;
-          return;
+          break;
         }
         ++rline;
       }
     }
-    // Didn't find the line
-  }
+    if (mLine != line_end) {
+      *aFoundValidLine = true;
+      if (mLine != cursor) {
+        aFrame->Properties().Set(nsBlockFrame::LineCursorProperty(), mLine);
+      }
+      return;
+    }
+  } else {
+    for (mLine = aFrame->begin_lines(); mLine != line_end; ++mLine) {
+      if (mLine->Contains(child)) {
+        *aFoundValidLine = true;
+        return;
+      }
+    }
+  }
+  // Didn't find the line
+  MOZ_ASSERT(mLine == line_end, "mLine should be line_end at this point");
 
   // If we reach here, it means that we have not been able to find the
   // desired frame in our in-flow lines.  So we should start looking at
   // our overflow lines. In order to do that, we set mLine to the end
   // iterator so that FindValidLine starts to look at overflow lines,
   // if any.
 
-  mLine = aFrame->end_lines();
-
   if (!FindValidLine())
     return;
 
   do {
     if (mLine->Contains(child)) {
       *aFoundValidLine = true;
       return;
     }

--- a/layout/generic/nsBlockFrame.h
+++ b/layout/generic/nsBlockFrame.h
@@ -167,18 +167,19 @@ public:
   void ClearLineCursor();
   // Get the first line that might contain y-coord 'y', or nullptr if you must search
   // all lines. If nonnull is returned then we guarantee that the lines'
   // combinedArea.ys and combinedArea.yMosts are non-decreasing.
   // The actual line returned might not contain 'y', but if not, it is guaranteed
   // to be before any line which does contain 'y'.
   nsLineBox* GetFirstLineContaining(nscoord y);
   // Set the line cursor to our first line. Only call this if you
-  // guarantee that the lines' combinedArea.ys and combinedArea.yMosts
-  // are non-decreasing.
+  // guarantee that either the lines' combinedArea.ys and combinedArea.
+  // yMosts are non-decreasing, or the line cursor is cleared before
+  // building the display list of this frame.
   void SetupLineCursor();
 
   virtual void ChildIsDirty(nsIFrame* aChild) override;
   virtual bool IsVisibleInSelection(nsISelection* aSelection) override;
 
   virtual bool IsEmpty() override;
   virtual bool CachedIsEmpty() override;
   virtual bool IsSelfEmpty() override;