Support dragging panels to stack and snap.

chrome/browser/ui/panels/panel_drag_controller.cc

Index: chrome/browser/ui/panels/panel_drag_controller.cc
diff --git a/chrome/browser/ui/panels/panel_drag_controller.cc b/chrome/browser/ui/panels/panel_drag_controller.cc
index d054ccf125b42ad085ef8bd2b37dddd2a327e3ff..260f2349ebaa75d89344739d5ed4c6b379248bcd 100644
--- a/chrome/browser/ui/panels/panel_drag_controller.cc
+++ b/chrome/browser/ui/panels/panel_drag_controller.cc
@@ -10,7 +10,6 @@
 #include "chrome/browser/ui/panels/docked_panel_collection.h"
 #include "chrome/browser/ui/panels/docked_panel_drag_handler.h"
 #include "chrome/browser/ui/panels/panel.h"
-#include "chrome/browser/ui/panels/panel_collection.h"
 #include "chrome/browser/ui/panels/panel_manager.h"
 #include "chrome/browser/ui/panels/stacked_panel_collection.h"
 #include "chrome/browser/ui/panels/stacked_panel_drag_handler.h"
@@ -25,6 +24,76 @@ const int kDetachDockedPanelThreshold = 100;
 // the docked area such that the detached panel becomes docked.
 const int kDockDetachedPanelThreshold = 30;
 
+// The minimum distance between two panels such that they can be stacked/snapped
+// together. This is one of two criteria needed for both stacking and snapping.

[Dmitry Titov, 2013/01/14 23:28:40]

"This is one of two criteria needed for both stacking and snapping." is not
clear - why it is important to say that? Maybe remove this sentence?

[jianli, 2013/01/15 01:43:43]

This is added per dcheng's comment. To make it more clear, I removed this extra
comment and shared one comment for both constants.

+const int kGluePanelsDistanceThreshold = 15;
+
+// The minimum overlap, in pixels, between two panels such that they can be
+// stacked/snapped together. This is one of two criteria needed for both
+// stacking and snapping.
+const int kGluePanelsOverlapThreshold = 10;
+
+int GetHorizontalOverlap(const gfx::Rect& bounds1, const gfx::Rect& bounds2) {
+  // Check for no overlap.
+  if (bounds1.right() <= bounds2.x() || bounds1.x() >= bounds2.right())
+    return 0;
+
+  // Check for complete overlap.
+  if (bounds2.x() <= bounds1.x() && bounds1.right() <= bounds2.right())
+    return bounds1.width();
+
+  if (bounds1.x() <= bounds2.x() && bounds2.right() <= bounds1.right())
+    return bounds2.width();
+
+  // Compute the overlap part.
+  return (bounds1.x() < bounds2.x()) ? (bounds1.right() - bounds2.x())
+                                     : (bounds2.right() - bounds1.x());
+}
+
+int GetVerticalOverlap(const gfx::Rect& bounds1, const gfx::Rect& bounds2) {
+  // Check for no overlap.
+  if (bounds1.bottom() <= bounds2.y() || bounds1.y() >= bounds2.bottom())
+    return 0;
+
+  // Check for complete overlap.
+  if (bounds2.y() <= bounds1.y() && bounds1.bottom() <= bounds2.bottom())
+    return bounds1.height();
+
+  if (bounds1.y() <= bounds2.y() && bounds2.bottom() <= bounds1.bottom())
+    return bounds2.height();
+
+  // Compute the overlap part.
+  return (bounds1.y() < bounds2.y()) ? (bounds1.bottom() - bounds2.y())
+                                     : (bounds2.bottom() - bounds1.y());
+}
+
+int GetVerticalDistance(const gfx::Rect& top_bounds,

[Dmitry Titov, 2013/01/14 23:28:40]

these functions use top_bounds/bottom_bounds while the functions above use
bounds1/bounds2.
Can we use bounds1/2 here as well? Otherwise, if the functions do expect
right>=left, then it's unclear why you need abs()...

[jianli, 2013/01/15 01:43:43]

We do need abs because bottom_bounds.y() might be above or below
top_bounds.bottom(). We can't use bounds1 and bounds2 since this function is to
compute the distance between the bottom edge of |top_bounds| and the top edge of
|bottom_bounds|. Added comment.

+                        const gfx::Rect& bottom_bounds) {
+  return abs(bottom_bounds.y() - top_bounds.bottom());
+}
+
+int GetHorizontalDistance(const gfx::Rect& left_bounds,
+                          const gfx::Rect& right_bounds) {
+  return abs(right_bounds.x() - left_bounds.right());
+}
+
+void SetPreviewModeForPanelAndBelow(Panel* panel, bool in_preview) {
+  StackedPanelCollection* stack = panel->stack();
+  if (stack) {
+    StackedPanelCollection::Panels::const_iterator iter =
+        stack->panels().begin();
+    for (; iter != stack->panels().end(); ++iter)
+      if (*iter == panel)

[Dmitry Titov, 2013/01/14 23:28:40]

lets put the body of this for() into {}, it'll read better this way.
Alternatively, it's probably better to have a single loop and a bool,
"panel_and_below", initially false, then becoming true as iter crosses panel,
then having an if(panel_and_below) {...}

[jianli, 2013/01/15 01:43:43]

Done.

+        break;
+    DCHECK(iter != stack->panels().end());
+    for (; iter != stack->panels().end(); ++iter)
+      if (in_preview != (*iter)->in_preview_mode())
+        (*iter)->SetPreviewMode(in_preview);
+  } else {
+    panel->SetPreviewMode(in_preview);
+  }
+}
+
 }  // namespace
 
 // static
@@ -37,8 +106,14 @@ int PanelDragController::GetDockDetachedPanelThresholdForTesting() {
   return kDockDetachedPanelThreshold;
 }
 
+// static
+int PanelDragController::GetGluePanelDistanceThresholdForTesting() {
+  return kGluePanelsDistanceThreshold;
+}
+
 PanelDragController::PanelDragController(PanelManager* panel_manager)
     : panel_manager_(panel_manager),
+      panel_stacking_enabled_(PanelManager::IsPanelStackingEnabled()),
       dragging_panel_(NULL),
       dragging_panel_original_collection_(NULL) {
 }
@@ -54,7 +129,7 @@ void PanelDragController::StartDragging(Panel* panel,
       mouse_location - panel->GetBounds().origin();
 
   dragging_panel_ = panel;
-  dragging_panel_->SetPreviewMode(true);
+  SetPreviewModeForPanelAndBelow(dragging_panel_, true);
 
   // Keep track of original collection and placement for the case that the drag
   // is cancelled.
@@ -68,12 +143,28 @@ void PanelDragController::Drag(const gfx::Point& mouse_location) {
 
   gfx::Point target_position = GetPanelPositionForMouseLocation(mouse_location);
 
+  if (panel_stacking_enabled_) {
+    // Check if the dragging panel can be moved out the stack. Note that this
+    // has to been done first and we should continue processing it for the case

[Dmitry Titov, 2013/01/14 23:28:40]

"to been" -> "to be"

[jianli, 2013/01/15 01:43:43]

Done.

+    // that the drag also triggers stacking and docking.
+    if (!TryUnstackFromTop(target_position))

[Dmitry Titov, 2013/01/14 23:28:40]

Why it is important to have this if() here instead of calling those
sequentially? If needed, comment would be good.

[jianli, 2013/01/15 01:43:43]

Certainly we call these two TryUnstack*** sequentially. But since only one of
them might be applicable, I thought using if was a bit small optimization. Added
comment.

+      TryUnstackFromBottom(target_position);
+
+    // Check if the dragging panel can stack with other panel or stack.
+    if (TryStack(target_position))
+      return;
+  }
+
   // Check if the dragging panel can be detached or docked.
   if (TryDetach(target_position))
     return;
   if (TryDock(target_position))
     return;
 
+  // Check if the dragging panel can snap to other panel.
+  if (panel_stacking_enabled_)
+    TrySnap(&target_position);

[Dmitry Titov, 2013/01/14 23:28:40]

It is possible that TryDetach() will succeed and we immediately can snap.
However we return before tryign to snap...

[jianli, 2013/01/15 01:43:43]

Good thought. Done.

+
   // At last, handle the drag via its collection's specific handler.
   switch (dragging_panel_->collection()->type()) {
     case PanelCollection::DOCKED:
@@ -83,7 +174,10 @@ void PanelDragController::Drag(const gfx::Point& mouse_location) {
       DetachedPanelDragHandler::HandleDrag(dragging_panel_, target_position);
       break;
     case PanelCollection::STACKED:
-      StackedPanelDragHandler::HandleDrag(dragging_panel_, target_position);
+      StackedPanelDragHandler::HandleDrag(
+          dragging_panel_,
+          target_position,
+          dragging_panel_->collection() == dragging_panel_original_collection_);
       break;
     default:
       NOTREACHED();
@@ -105,14 +199,13 @@ void PanelDragController::EndDragging(bool cancelled) {
           static_cast<PanelCollection::PositioningMask>(
               PanelCollection::DEFAULT_POSITION |
               PanelCollection::DO_NOT_UPDATE_BOUNDS);
-      panel_manager_->MovePanelToCollection(
-          dragging_panel_,
-          dragging_panel_original_collection_,
-          positioning_mask);
+      MovePanelAndBelowToCollection(dragging_panel_,
+                                    dragging_panel_original_collection_,
+                                    positioning_mask);
     }
 
     // End the preview mode.
-    dragging_panel_->SetPreviewMode(false);
+    SetPreviewModeForPanelAndBelow(dragging_panel_, false);
 
     // Restore the dragging panel to its original placement.
     dragging_panel_original_collection_->RestorePanelToSavedPlacement();
@@ -120,13 +213,26 @@ void PanelDragController::EndDragging(bool cancelled) {
     // The saved placement is no longer needed.
     dragging_panel_original_collection_->DiscardSavedPanelPlacement();
 
+    // Finalizing the drag is only needed for the stack.
+    if (current_collection->type() == PanelCollection::STACKED)
+      StackedPanelDragHandler::FinalizeDrag(dragging_panel_);
+
     // End the preview mode.
-    dragging_panel_->SetPreviewMode(false);
+    SetPreviewModeForPanelAndBelow(dragging_panel_, false);
 
     // This could cause the panel to be moved to its finalized position.
     current_collection->RefreshLayout();
   }
 
+  // If the origianl collection is a stack and it becomes empty, remove it.
+  if (dragging_panel_original_collection_->type() == PanelCollection::STACKED) {
+    StackedPanelCollection* original_stack =
+        static_cast<StackedPanelCollection*>(
+            dragging_panel_original_collection_);
+    if (original_stack->num_panels() == 0)
+      panel_manager_->RemoveStack(original_stack);
+  }
+
   dragging_panel_ = NULL;
 }
 
@@ -171,16 +277,16 @@ bool PanelDragController::TryDetach(const gfx::Point& target_position) {
     return false;
 
   // Panels in the detached collection are always at their full size.
-  gfx::Rect target_panel_bounds(target_position,
+  gfx::Rect target_bounds(target_position,
                                 dragging_panel_->full_size());
 
   // The panel should be dragged up high enough to pass certain threshold.
   if (panel_manager_->docked_collection()->display_area().bottom() -
-          target_panel_bounds.bottom() < kDetachDockedPanelThreshold)
+          target_bounds.bottom() < kDetachDockedPanelThreshold)
     return false;
 
   // Apply new panel bounds.
-  dragging_panel_->SetPanelBoundsInstantly(target_panel_bounds);
+  dragging_panel_->SetPanelBoundsInstantly(target_bounds);
 
   // Move the panel to new collection.
   panel_manager_->MovePanelToCollection(dragging_panel_,
@@ -195,7 +301,7 @@ bool PanelDragController::TryDock(const gfx::Point& target_position) {
   if (dragging_panel_->collection()->type() != PanelCollection::DETACHED)
     return false;
 
-  gfx::Rect target_panel_bounds(target_position,
+  gfx::Rect target_bounds(target_position,
                                 dragging_panel_->GetBounds().size());
 
   // If the target panel bounds is outside the main display area where the
@@ -203,17 +309,17 @@ bool PanelDragController::TryDock(const gfx::Point& target_position) {
   // be still free-floating.
   gfx::Rect display_area = panel_manager_->display_settings_provider()->
       GetDisplayArea();
-  if (!display_area.Intersects(target_panel_bounds))
+  if (!display_area.Intersects(target_bounds))
     return false;
 
   // The bottom of the panel should come very close to or fall below the bottom
   // of the docked area.
   if (panel_manager_->docked_collection()->display_area().bottom() -
-         target_panel_bounds.bottom() > kDockDetachedPanelThreshold)
+         target_bounds.bottom() > kDockDetachedPanelThreshold)
     return false;
 
   // Apply new panel bounds.
-  dragging_panel_->SetPanelBoundsInstantly(target_panel_bounds);
+  dragging_panel_->SetPanelBoundsInstantly(target_bounds);
 
   // Move the panel to new collection.
   panel_manager_->MovePanelToCollection(dragging_panel_,
@@ -222,3 +328,420 @@ bool PanelDragController::TryDock(const gfx::Point& target_position) {
 
   return true;
 }
+
+bool PanelDragController::TryStack(const gfx::Point& target_position) {
+  gfx::Rect target_bounds;
+  GlueEdge target_edge;
+  Panel* target_panel = FindPanelToGlue(target_position,
+                                        STACK,
+                                        &target_bounds,
+                                        &target_edge);
+  if (!target_panel)
+    return false;
+
+  StackedPanelCollection* dragging_stack = dragging_panel_->stack();
+
+  // Move the panel (and all the panels below if in a stack) to the new
+  // position.
+  gfx::Vector2d delta_origin =

[Dmitry Titov, 2013/01/14 23:28:40]

lets just call it 'delta'. I understand you meant 'the delta by which the origin
will move' but it also can be read like 'origin of delta' which is confusing...

Same for couple of places below which use delta_origin.

[jianli, 2013/01/15 01:43:43]

Done.

+      target_bounds.origin() - dragging_panel_->GetBounds().origin();
+  if (dragging_stack)
+    dragging_stack->MoveAllDraggingPanelsInstantly(delta_origin);
+  else
+    dragging_panel_->MoveByInstantly(delta_origin);
+
+  // If the panel to stack with is not in a stack, create it now.
+  StackedPanelCollection* target_stack = target_panel->stack();
+  if (!target_stack) {
+    target_stack = panel_manager_->CreateStack();
+    panel_manager_->MovePanelToCollection(target_panel,
+                                          target_stack,
+                                          PanelCollection::DEFAULT_POSITION);
+  }
+
+  // Move the panel to new collection.
+  // Note that we don't want to refresh the layout now because when we add
+  // a panel to top of other panel, we don't want the bottom panel to change
+  // its width to be same as top panel now.
+  PanelCollection::PositioningMask positioning_mask =
+      static_cast<PanelCollection::PositioningMask>(
+          PanelCollection::NO_LAYOUT_REFRESH |
+          (target_edge == TOP_EDGE ? PanelCollection::TOP_POSITION
+                                   : PanelCollection::DEFAULT_POSITION));
+  MovePanelAndBelowToCollection(dragging_panel_,
+                                target_stack,
+                                positioning_mask);
+
+  return true;
+}
+
+bool PanelDragController::TryUnstackFromTop(const gfx::Point& target_position) {

[Dmitry Titov, 2013/01/14 23:28:40]

Lets add a comment for this function, what exactly is "UnstackingFromTop". Same
for "UnstackingFromBottom".

[jianli, 2013/01/15 01:43:43]

Done.

+  // It has to be stacked.
+  StackedPanelCollection* dragging_stack = dragging_panel_->stack();
+  if (!dragging_stack)
+    return false;
+
+  // Unstacking from top only happens when a panel/stack stacks to the top of
+  // another panel and then moves away while the drag is still in progress.
+  if (dragging_panel_ != dragging_stack->top_panel() ||
+      dragging_stack == dragging_panel_original_collection_)
+    return false;
+
+  // Count the number of panels that might need to unstack.
+  Panel* last_panel_to_unstack = NULL;
+  Panel* panel_below_last_panel_to_unstack = NULL;
+  int num_panels_to_unstack = 0;
+  for (StackedPanelCollection::Panels::const_iterator iter =
+           dragging_stack->panels().begin();
+       iter != dragging_stack->panels().end(); ++iter) {
+    if (!(*iter)->in_preview_mode()) {
+      panel_below_last_panel_to_unstack = *iter;
+      break;
+    }
+    num_panels_to_unstack++;
+    last_panel_to_unstack = *iter;
+  }
+  DCHECK_GE(num_panels_to_unstack, 1);
+  if (num_panels_to_unstack == dragging_stack->num_panels())
+    return false;
+
+  gfx::Vector2d delta_origin =
+      target_position - dragging_panel_->GetBounds().origin();
+
+  // The last panel to unstack should be dragged far enough from its below
+  // panel.
+  gfx::Rect target_bounds = last_panel_to_unstack->GetBounds();
+  target_bounds.Offset(delta_origin);
+  gfx::Rect below_panel_bounds = panel_below_last_panel_to_unstack->GetBounds();
+  if (GetVerticalDistance(target_bounds, below_panel_bounds) <
+          kGluePanelsDistanceThreshold &&
+      GetHorizontalOverlap(target_bounds, below_panel_bounds) >
+          kGluePanelsOverlapThreshold) {
+    return false;
+  }
+
+  // Move the panel (and all the panels below if in a stack) to the new
+  // position.
+  for (StackedPanelCollection::Panels::const_iterator iter =
+           dragging_stack->panels().begin();
+       iter != dragging_stack->panels().end(); ++iter) {
+    if (!(*iter)->in_preview_mode())
+      break;
+    (*iter)->MoveByInstantly(delta_origin);
+  }
+
+  int num_panels_in_stack = dragging_stack->num_panels();
+  DCHECK_GE(num_panels_in_stack, 2);
+
+  DetachedPanelCollection* detached_collection =

[Dmitry Titov, 2013/01/14 23:28:40]

Here there is an assumption that unstaking happens always into
DetachedPanelCollection. But it also can be moved right into Dock... I think
it's ok because you will check TryDock later anyways, but maybe a comment here
would help.

[jianli, 2013/01/15 01:43:43]

Done.

+      panel_manager_->detached_collection();
+
+  // If there're only 2 panels in the stack, both panels should move out of the
+  // stack and the stack should be removed.
+  if (num_panels_in_stack == 2) {
+    DCHECK_EQ(1, num_panels_to_unstack);
+    MovePanelAndBelowToCollection(dragging_panel_,
+                                  detached_collection,
+                                  PanelCollection::KNOWN_POSITION);
+    return true;
+  }
+
+  DCHECK_GE(num_panels_in_stack, 3);
+
+  // If only one panel (top panel) needs to unstack, move it out of the stack.
+  if (num_panels_to_unstack == 1) {
+    panel_manager_->MovePanelToCollection(dragging_panel_,
+                                          panel_manager_->detached_collection(),
+                                          PanelCollection::KNOWN_POSITION);
+    return true;
+  }
+
+  // If all the panels except the bottom panel need to unstack, simply move
+  // bottom panel out of the stack.
+  if (num_panels_in_stack - num_panels_to_unstack == 1) {
+    panel_manager_->MovePanelToCollection(dragging_stack->bottom_panel(),
+                                          panel_manager_->detached_collection(),
+                                          PanelCollection::KNOWN_POSITION);
+    return true;
+  }
+
+  // Otherwise, move all unstacked panels to a new stack.
+  // Note that all the panels to move should be copied to a local list first
+  // because the stack collection will be modified during the move.
+  std::list<Panel*> panels_to_move;
+  for (StackedPanelCollection::Panels::const_iterator iter =
+           dragging_stack->panels().begin();
+       iter != dragging_stack->panels().end(); ++iter) {
+    Panel* panel = *iter;
+    if (!panel->in_preview_mode())
+      break;
+    panels_to_move.push_back(panel);
+  }
+  StackedPanelCollection* new_stack = panel_manager_->CreateStack();
+  for (std::list<Panel*>::const_iterator iter = panels_to_move.begin();
+       iter != panels_to_move.end(); ++iter) {
+    panel_manager_->MovePanelToCollection(*iter,
+                                          new_stack,
+                                          PanelCollection::KNOWN_POSITION);
+  }
+
+  return true;
+}
+
+bool PanelDragController::TryUnstackFromBottom(
+    const gfx::Point& target_position) {
+  // It has to be stacked.
+  StackedPanelCollection* dragging_stack = dragging_panel_->stack();
+  if (!dragging_stack)
+    return false;
+
+  // It cannot be the top panel.
+  if (dragging_panel_ == dragging_stack->top_panel())
+    return false;
+
+  DCHECK_GT(dragging_stack->num_panels(), 1);
+
+  gfx::Rect target_bounds(target_position, dragging_panel_->GetBounds().size());
+
+  // The panel should be dragged far enough from its above panel.
+  Panel* above_panel = dragging_stack->GetPanelAbove(dragging_panel_);
+  DCHECK(above_panel);
+  gfx::Rect above_panel_bounds = above_panel->GetBounds();
+  if (GetVerticalDistance(above_panel_bounds, target_bounds) <
+          kGluePanelsDistanceThreshold &&
+      GetHorizontalOverlap(above_panel_bounds, target_bounds) >
+          kGluePanelsOverlapThreshold) {
+    return false;
+  }
+
+  // Move the panel (and all the panels below if in a stack) to the new
+  // position.
+  gfx::Vector2d delta_origin =
+      target_position - dragging_panel_->GetBounds().origin();
+  if (dragging_stack)
+    dragging_stack->MoveAllDraggingPanelsInstantly(delta_origin);
+  else
+    dragging_panel_->MoveByInstantly(delta_origin);
+
+  // If there're only 2 panels in the stack, both panels should move out the
+  // stack and the stack should be removed.
+  DetachedPanelCollection* detached_collection =
+      panel_manager_->detached_collection();
+  if (dragging_stack->num_panels() == 2) {
+    MovePanelAndBelowToCollection(dragging_stack->top_panel(),
+                                  detached_collection,
+                                  PanelCollection::KNOWN_POSITION);
+    return true;
+  }
+
+  // There're at least 3 panels.
+  DCHECK_GE(dragging_stack->num_panels(), 3);
+
+  // If the dragging panel is bottom panel, move it out of the stack.
+  if (dragging_panel_ == dragging_stack->bottom_panel()) {
+    panel_manager_->MovePanelToCollection(dragging_panel_,
+                                          detached_collection,
+                                          PanelCollection::KNOWN_POSITION);
+    return true;
+  }
+
+  // If the dragging panel is the one below the top panel, move top panel
+  // out of the stack.
+  if (dragging_stack->GetPanelAbove(dragging_panel_) ==
+      dragging_stack->top_panel()) {
+    panel_manager_->MovePanelToCollection(dragging_stack->top_panel(),
+                                          detached_collection,
+                                          PanelCollection::KNOWN_POSITION);
+    return true;
+  }
+
+  // There're at least 4 panels.
+  DCHECK_GE(dragging_stack->num_panels(), 4);
+
+  // We can split them into 2 stacks by moving the dragging panel and all panels
+  // below to a new stack while keeping all panels above in the same stack.
+  StackedPanelCollection* new_stack = panel_manager_->CreateStack();
+  MovePanelAndBelowToCollection(dragging_panel_,
+                                new_stack,
+                                PanelCollection::KNOWN_POSITION);
+
+  return true;
+}
+
+void PanelDragController::TrySnap(gfx::Point* target_position) {
+  // Snapping does not apply to docked panels.
+  if (dragging_panel_->collection()->type() == PanelCollection::DOCKED)
+    return;
+
+  gfx::Rect target_bounds;
+  GlueEdge target_edge;
+  Panel* target_panel = FindPanelToGlue(*target_position,
+                                        SNAP,
+                                        &target_bounds,
+                                        &target_edge);
+  if (!target_panel)
+    return;
+
+  *target_position = target_bounds.origin();
+}
+
+Panel* PanelDragController::FindPanelToGlue(
+    const gfx::Point& potential_position,
+    GlueAction action,
+    gfx::Rect* target_bounds,
+    GlueEdge* target_edge) const {
+  int best_distance = kint32max;
+  Panel* best_matching_panel = NULL;
+
+  // Compute the potential bounds for the dragging panel.
+  gfx::Rect current_dragging_bounds = dragging_panel_->GetBounds();
+  gfx::Rect potential_dragging_bounds(potential_position,
+                                      current_dragging_bounds.size());
+
+  // Compute the potential bounds for the bottom panel if the dragging panel is
+  // in a stack. If not, it is same as |potential_dragging_bounds|.
+  // This is used to determine if the dragging panel or the bottom panel can
+  // stack to the top of other panel.
+  gfx::Rect current_bottom_bounds;
+  gfx::Rect potential_bottom_bounds;
+  StackedPanelCollection* dragging_stack = dragging_panel_->stack();
+  if (dragging_stack && dragging_panel_ != dragging_stack->bottom_panel()) {
+    gfx::Vector2d delta = potential_position - current_dragging_bounds.origin();
+    current_bottom_bounds = dragging_stack->bottom_panel()->GetBounds();
+    potential_bottom_bounds = current_bottom_bounds;
+    potential_bottom_bounds.Offset(delta);
+  } else {
+    current_bottom_bounds = current_dragging_bounds;
+    potential_bottom_bounds = potential_dragging_bounds;
+  }
+
+  // Go through all non-docked panels.
+  std::vector<Panel*> panels = panel_manager_->GetDetachedAndStackedPanels();
+  for (std::vector<Panel*>::const_iterator iter = panels.begin();
+       iter != panels.end(); ++iter) {
+    Panel* panel = *iter;
+    if (dragging_panel_ == panel)
+      continue;
+    if (dragging_panel_->collection()->type() == PanelCollection::STACKED &&
+        dragging_panel_->collection() == panel->collection())
+      continue;
+    gfx::Rect panel_bounds = panel->GetBounds();
+    int distance;
+    int overlap;
+
+    if (action == SNAP) {
+      overlap = GetVerticalOverlap(potential_dragging_bounds, panel_bounds);
+      if (overlap > kGluePanelsOverlapThreshold) {
+        // Can |dragging_panel_| snap to left edge of |panel|?
+        distance = GetHorizontalDistance(potential_dragging_bounds,
+                                         panel_bounds);
+        if (distance < kGluePanelsDistanceThreshold &&
+            distance < best_distance) {
+          best_distance = distance;
+          best_matching_panel = panel;
+          *target_edge = LEFT_EDGE;
+          *target_bounds = potential_dragging_bounds;
+          target_bounds->set_x(panel_bounds.x() - target_bounds->width());
+        }
+
+        // Can |dragging_panel_| snap to right edge of |panel|?
+        distance = GetHorizontalDistance(panel_bounds,
+                                         potential_dragging_bounds);
+        if (distance < kGluePanelsDistanceThreshold &&
+            distance < best_distance) {
+          best_distance = distance;
+          best_matching_panel = panel;
+          *target_edge = RIGHT_EDGE;
+          *target_bounds = potential_dragging_bounds;
+          target_bounds->set_x(panel_bounds.right());
+        }
+      }
+    } else {
+      DCHECK_EQ(STACK, action);
+
+      // Can |dragging_panel_| or the bottom panel in |dragging_panel_|'s stack
+      // stack to top edge of |panel|?
+      distance = GetVerticalDistance(potential_bottom_bounds, panel_bounds);
+      overlap = GetHorizontalOverlap(panel_bounds, potential_bottom_bounds);
+      if (distance < kGluePanelsDistanceThreshold &&
+          overlap > kGluePanelsOverlapThreshold &&
+          distance < best_distance) {
+        best_distance = distance;
+        best_matching_panel = panel;
+        *target_edge = TOP_EDGE;
+        target_bounds->SetRect(
+            potential_dragging_bounds.x(),
+            current_dragging_bounds.y() + panel_bounds.y() -
+                current_bottom_bounds.height() - current_bottom_bounds.y(),
+            potential_dragging_bounds.width(),
+            potential_dragging_bounds.height());
+      }
+
+      // Can |dragging_panel_| stack to bottom edge of |panel|?
+      distance = GetVerticalDistance(panel_bounds, potential_dragging_bounds);
+      overlap = GetHorizontalOverlap(panel_bounds, potential_dragging_bounds);
+      if (distance < kGluePanelsDistanceThreshold &&
+          overlap > kGluePanelsOverlapThreshold &&
+          distance < best_distance) {
+        best_distance = distance;
+        best_matching_panel = panel;
+        *target_edge = BOTTOM_EDGE;
+        target_bounds->SetRect(potential_dragging_bounds.x(),
+                               panel_bounds.bottom(),
+                               potential_dragging_bounds.width(),
+                               potential_dragging_bounds.height());
+      }
+    }
+  }
+
+  return best_matching_panel;
+}
+
+void PanelDragController::MovePanelAndBelowToCollection(
+    Panel* panel,
+    PanelCollection* target_collection,
+    PanelCollection::PositioningMask positioning_mask) const {
+  StackedPanelCollection* stack = panel->stack();
+  if (!stack) {
+    panel_manager_->MovePanelToCollection(panel,
+                                          target_collection,
+                                          positioning_mask);
+    return;
+  }
+
+  // Note that all the panels to move should be copied to a local list first
+  // because the stack collection will be modified during the move.
+  std::list<Panel*> panels_to_move;
+  StackedPanelCollection::Panels::const_iterator iter = stack->panels().begin();
+  for (; iter != stack->panels().end(); ++iter)
+    if ((*iter) == panel)
+      break;
+  for (; iter != stack->panels().end(); ++iter) {
+    // Note that if the panels are going to be inserted from the top, we need
+    // to reverse the order when copying to the local list.
+    if (positioning_mask & PanelCollection::TOP_POSITION)
+      panels_to_move.push_front(*iter);
+    else
+      panels_to_move.push_back(*iter);
+  }
+  for (std::list<Panel*>::const_iterator panel_iter = panels_to_move.begin();
+       panel_iter != panels_to_move.end(); ++panel_iter) {
+    panel_manager_->MovePanelToCollection(*panel_iter,
+                                          target_collection,
+                                          positioning_mask);
+  }
+
+  // If the stack becomes empty or has only one panel left, no need to keep
+  // the stack.
+  if (stack && stack->num_panels() <= 1) {
+    if (stack->num_panels() == 1) {
+      panel_manager_->MovePanelToCollection(
+          stack->top_panel(),
+          panel_manager_->detached_collection(),
+          PanelCollection::KNOWN_POSITION);
+    }
+    // Note that if the stack is the original collection, do not remove it now.

[Dmitry Titov, 2013/01/14 23:28:40]

Why? could be useful to add a reason to keep it, for a better comment

[jianli, 2013/01/15 01:43:43]

Done.

+    if (stack != dragging_panel_original_collection_)
+      panel_manager_->RemoveStack(stack);
+  }
+}