Tab dragging: Fix the regression caused by the refactoring.

It was introduced when rebasing this refactoring CL
https://chromium-review.googlesource.com/c/chromium/src/+/1134503.
Previously it was rebased against a child branch (which was later
discarded), then then rebased against master branch, but some changes
from the child branch remained. The CL reverts the changes that's
introduced by the child branch. The refactoring CL should not do any
functionality changes.

Bug: None
Change-Id: I05af1fd5fc61be4f357106ae58404f9a2d2f5b0d
Reviewed-on: https://chromium-review.googlesource.com/1141112
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576113}

ash/wm/tablet_mode/tablet_mode_browser_window_drag_delegate.cc

@@ -174,17 +174,6 @@ void TabletModeBrowserWindowDragDelegate::UpdateSourceWindow(
 
 void TabletModeBrowserWindowDragDelegate::UpdateScrim(
     const gfx::Point& location_in_screen) {
-  // If overview mode is active, do not show the scrim on the overview side of
-  // the screen.
-  if (Shell::Get()->window_selector_controller()->IsSelecting()) {
-    WindowGrid* window_grid = GetWindowSelector()->GetGridWithRootWindow(
-        dragged_window_->GetRootWindow());
-    if (window_grid && window_grid->bounds().Contains(location_in_screen)) {
-      scrim_.reset();
-      return;
-    }
-  }
-
   const gfx::Rect work_area_bounds =
       display::Screen::GetScreen()
           ->GetDisplayNearestWindow(dragged_window_)
@@ -197,23 +186,19 @@ void TabletModeBrowserWindowDragDelegate::UpdateScrim(
     return;
   }
 
-  SplitViewController::SnapPosition snap_position =
-      GetSnapPosition(location_in_screen);
-
-  bool should_show_blurred_scrim = false;
-  if (location_in_screen.y() >=
-      GetMaximizeVerticalThreshold(work_area_bounds)) {
-    if (split_view_controller_->IsSplitViewModeActive() !=
-        (snap_position == SplitViewController::NONE)) {
-      should_show_blurred_scrim = true;
-    }
-  }
-
-  if (!should_show_blurred_scrim) {
+  // If overview mode is active, do not show the scrim on the overview side of
+  // the screen.
+  if (Shell::Get()->window_selector_controller()->IsSelecting()) {
+    WindowGrid* window_grid = GetWindowSelector()->GetGridWithRootWindow(
+        dragged_window_->GetRootWindow());
+    if (window_grid && window_grid->bounds().Contains(location_in_screen)) {
       scrim_.reset();
       return;
     }
+  }
 
+  SplitViewController::SnapPosition snap_position =
+      GetSnapPosition(location_in_screen);
   gfx::Rect expected_bounds(work_area_bounds);
   if (split_view_controller_->IsSplitViewModeActive()) {
     expected_bounds = split_view_controller_->GetSnappedWindowBoundsInScreen(
@@ -223,7 +208,21 @@ void TabletModeBrowserWindowDragDelegate::UpdateScrim(
                           kHighlightScreenEdgePaddingDp);
   }
 
-  ShowScrim(kScrimOpacity, kScrimBlur, expected_bounds);
+  bool should_show_blurred_scrim = false;
+  if (location_in_screen.y() >=
+      TabletModeWindowDragDelegate::GetMaximizeVerticalThreshold(
+          work_area_bounds)) {
+    if (split_view_controller_->IsSplitViewModeActive() !=
+        (snap_position == SplitViewController::NONE)) {
+      should_show_blurred_scrim = true;
+    }
+  }
+  // When the event is between |indicators_vertical_threshold| and
+  // |maximize_vertical_threshold|, the scrim is still shown but is invisible
+  // to the user (transparent). It's needed to prevent the dragged window to
+  // merge into the scaled down source window.
+  ShowScrim(should_show_blurred_scrim ? kScrimOpacity : 0.f,
+            should_show_blurred_scrim ? kScrimBlur : 0.f, expected_bounds);
 }
 
 void TabletModeBrowserWindowDragDelegate::ShowScrim(