Bug 849524 - Avoid reflowing when <input type=range>'s thumb position needs to be updated due to its value changing. r=dholbert

content/html/content/src/nsHTMLInputElement.cpp

@@ -2599,11 +2599,9 @@ nsHTMLInputElement::SetValueOfRangeForUserEvent(double aValue)
   nsAutoString val;
   ConvertNumberToString(aValue, val);
   SetValueInternal(val, true, true);
-  nsIFrame* frame = GetPrimaryFrame();
+  nsRangeFrame* frame = do_QueryFrame(GetPrimaryFrame());
   if (frame) {
-    // Trigger reflow to update the position of the thumb:
-    frame->PresContext()->GetPresShell()->
-      FrameNeedsReflow(frame, nsIPresShell::eResize, NS_FRAME_IS_DIRTY);
+    frame->UpdateThumbPositionForValueChange();
   }
 }
 

layout/forms/nsRangeFrame.cpp

@@ -378,6 +378,34 @@ nsRangeFrame::GetValueAtEventPoint(nsGUIEvent* aEvent)
   return minimum + (double(contentPoint.x) / double(contentRect.width)) * range;
 }
 
+void
+nsRangeFrame::UpdateThumbPositionForValueChange()
+{
+  if (NS_SUBTREE_DIRTY(this)) {
+    return; // we're going to be updated when we reflow
+  }
+  nsIFrame* thumbFrame = mThumbDiv->GetPrimaryFrame();
+  if (!thumbFrame) {
+    return; // diplay:none?
+  }
+  // TODO in bug 842179 - factor out duplication here and in Reflow.
+  double fraction = GetValueAsFractionOfRange();
+  nsRect contentRect = GetContentRectRelativeToSelf();
+  nsMargin borderAndPadding = GetUsedBorderAndPadding();
+  nsSize thumbSize = thumbFrame->GetSize();
+  nsPoint newPosition(borderAndPadding.left, borderAndPadding.top);
+  if (IsHorizontal()) {
+    newPosition += nsPoint(NSToCoordRound(fraction * contentRect.width) -
+                             thumbSize.width/2,
+                           (contentRect.height - thumbSize.height)/2);
+  } else {
+    newPosition += nsPoint((contentRect.width - thumbSize.width)/2,
+                           NSToCoordRound(fraction * contentRect.height) -
+                             thumbSize.height/2);
+  }
+  thumbFrame->SetPosition(newPosition);
+  SchedulePaint();
+}
 
 NS_IMETHODIMP
 nsRangeFrame::AttributeChanged(int32_t  aNameSpaceID,
@@ -392,19 +420,7 @@ nsRangeFrame::AttributeChanged(int32_t  aNameSpaceID,
        aAttribute == nsGkAtoms::min ||
        aAttribute == nsGkAtoms::max ||
        aAttribute == nsGkAtoms::step)) {
-    nsIFrame* trackFrame = mTrackDiv->GetPrimaryFrame();
-    if (trackFrame) { // diplay:none?
-      PresContext()->PresShell()->FrameNeedsReflow(trackFrame,
-                                                   nsIPresShell::eResize,
-                                                   NS_FRAME_IS_DIRTY);
-    }
-
-    nsIFrame* thumbFrame = mThumbDiv->GetPrimaryFrame();
-    if (thumbFrame) { // diplay:none?
-      PresContext()->PresShell()->FrameNeedsReflow(thumbFrame,
-                                                   nsIPresShell::eResize,
-                                                   NS_FRAME_IS_DIRTY);
-    }
+    UpdateThumbPositionForValueChange();
   }
 
   return nsContainerFrame::AttributeChanged(aNameSpaceID, aAttribute, aModType);

layout/forms/nsRangeFrame.h

@@ -92,6 +92,13 @@ public:
 
   double GetValueAtEventPoint(nsGUIEvent* aEvent);
 
+  /**
+   * Helper to reposition the thumb and schedule a repaint when the value of
+   * the range changes. (This does not reflow, since the position and size of
+   * the thumb do not affect the position or size of any other frames.)
+   */
+  void UpdateThumbPositionForValueChange();
+
 private:
 
   // Helper function which reflows the anonymous div frames.