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..70309ccf8942494692e50a2cf708c6be9a5dbdd8 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 +24,78 @@ const int kDetachDockedPanelThreshold = 100;
 // the docked area such that the detached panel becomes docked.
 const int kDockDetachedPanelThreshold = 30;
 
+// The minimum distance and overlap (in pixels) between two panels such that
+// they can be stacked/snapped together.
+const int kGluePanelsDistanceThreshold = 15;
+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());
+}
+
+// Return the vertical distance between the bottom edge of |top_bounds| and
+// the top edge of |bottom_bounds|.
+int GetVerticalDistance(const gfx::Rect& top_bounds,
+                        const gfx::Rect& bottom_bounds) {
+  return abs(bottom_bounds.y() - top_bounds.bottom());
+}
+
+// Return the vertical distance between the right edge of |left_bounds| and
+// the left edge of |right_bounds|.
+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) {
+    bool panel_found = false;
+    for (StackedPanelCollection::Panels::const_iterator iter =
+             stack->panels().begin();
+         iter != stack->panels().end(); ++iter) {
+      Panel* current_panel = *iter;
+      if (!panel_found && current_panel != panel)
+        continue;
+      panel_found = true;
+      if (in_preview != current_panel->in_preview_mode())
+        current_panel->SetPreviewMode(in_preview);
+    }
+  } else {
+    panel->SetPreviewMode(in_preview);
+  }
+}
+
 }  // namespace
 
 // static
@@ -37,8 +108,19 @@ int PanelDragController::GetDockDetachedPanelThresholdForTesting() {
   return kDockDetachedPanelThreshold;
 }
 
+// static
+int PanelDragController::GetGluePanelDistanceThresholdForTesting() {
+  return kGluePanelsDistanceThreshold;
+}
+
+// static
+int PanelDragController::GetGluePanelOverlapThresholdForTesting() {
+  return kGluePanelsOverlapThreshold;
+}
+
 PanelDragController::PanelDragController(PanelManager* panel_manager)
     : panel_manager_(panel_manager),
+      panel_stacking_enabled_(PanelManager::IsPanelStackingEnabled()),
       dragging_panel_(NULL),
       dragging_panel_original_collection_(NULL) {
 }
@@ -54,7 +136,7 @@ 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.
@@ -68,11 +150,29 @@ void PanelDragController::Drag(const gfx::Point& mouse_location) {
 
   gfx::Point target_position = GetPanelPositionForMouseLocation(mouse_location);
 
-  // Check if the dragging panel can be detached or docked.
-  if (TryDetach(target_position))
-    return;
-  if (TryDock(target_position))
-    return;
+  if (panel_stacking_enabled_) {
+    // Check if the dragging panel can be moved out the stack. Note that this
+    // has to be done first and we should continue processing it for the case
+    // that the drag also triggers stacking and docking.
+    // Note that the panel can only be unstacked from top or bottom. So if
+    // unstacking from top succeeds, there is no need to check for unstacking
+    // from bottom.
+    if (!TryUnstackFromTop(target_position))
+      TryUnstackFromBottom(target_position);
+
+    // Check if the dragging panel can stack with other panel or stack.
+    TryStack(target_position);
+  }
+
+  // Check if the dragging panel can be docked.
+  TryDock(target_position);
+
+  // Check if the dragging panel can be detached.
+  TryDetach(target_position);
+
+  // Check if the dragging panel can snap to other panel.
+  if (panel_stacking_enabled_)
+    TrySnap(&target_position);
 
   // At last, handle the drag via its collection's specific handler.
   switch (dragging_panel_->collection()->type()) {
@@ -83,7 +183,10 @@ 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 +208,13 @@ 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 +222,26 @@ 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;
 }
 
@@ -161,41 +276,39 @@ gfx::Point PanelDragController::GetPanelPositionForMouseLocation(
   return target_position;
 }
 
-bool PanelDragController::TryDetach(const gfx::Point& target_position) {
+void PanelDragController::TryDetach(const gfx::Point& target_position) {
   // It has to come from the docked collection.
   if (dragging_panel_->collection()->type() != PanelCollection::DOCKED)
-    return false;
+    return;
 
   // The minimized docked panel is not allowed to detach.
   if (dragging_panel_->IsMinimized())
-    return false;
+    return;
 
   // 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)
-    return false;
+          target_bounds.bottom() < kDetachDockedPanelThreshold)
+    return;
 
   // 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_,
                                         panel_manager_->detached_collection(),
                                         PanelCollection::KNOWN_POSITION);
-
-  return true;
 }
 
-bool PanelDragController::TryDock(const gfx::Point& target_position) {
+void PanelDragController::TryDock(const gfx::Point& target_position) {
   // It has to come from the detached collection.
   if (dragging_panel_->collection()->type() != PanelCollection::DETACHED)
-    return false;
+    return;
 
-  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
@@ -203,22 +316,444 @@ 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))
-    return false;
+  if (!display_area.Intersects(target_bounds))
+    return;
 
   // 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)
-    return false;
+         target_bounds.bottom() > kDockDetachedPanelThreshold)
+    return;
 
   // 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_,
                                         panel_manager_->docked_collection(),
                                         PanelCollection::KNOWN_POSITION);
+}
+
+void 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;
+
+  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 =
+      target_bounds.origin() - dragging_panel_->GetBounds().origin();
+  if (dragging_stack)
+    dragging_stack->MoveAllDraggingPanelsInstantly(delta);
+  else
+    dragging_panel_->MoveByInstantly(delta);
+
+  // 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);
+}
+
+// Check if a panel or a set of stacked panels (being dragged together from a
+// stack) can be dragged away from the panel below such that the former panel(s)
+// are not in the same stack as the latter panel.
+bool PanelDragController::TryUnstackFromTop(const gfx::Point& target_position) {
+  // 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 = 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);
+  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);
+  }
+
+  int num_panels_in_stack = dragging_stack->num_panels();
+  DCHECK_GE(num_panels_in_stack, 2);
+
+  // When a panel is removed from its stack, we always make it detached. If it
+  // indeed should go to the docked collection, the subsequent TryDock will then
+  // move it from the detached collection to the docked collection.
+  DetachedPanelCollection* detached_collection =
+      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;
+}
+
+// Check if a panel or a set of stacked panels (being dragged together from a
+// stack) can be dragged away from the panel above such that the former panel(s)
+// are not in the same stack as the latter panel.
+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 = target_position - dragging_panel_->GetBounds().origin();
+  if (dragging_stack)
+    dragging_stack->MoveAllDraggingPanelsInstantly(delta);
+  else
+    dragging_panel_->MoveByInstantly(delta);
+
+  // 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.
+    // This is because the original collection contains the information to
+    // restore the dragging panel to the right place when the drag is cancelled.
+    if (stack != dragging_panel_original_collection_)
+      panel_manager_->RemoveStack(stack);
+  }
+}