Reland: Force calling PostDisplayConfigurationChange() even if earyling out in UpdateDisplays

ash/display/display_manager.cc

Index: ash/display/display_manager.cc
diff --git a/ash/display/display_manager.cc b/ash/display/display_manager.cc
index 29eced4cc03ec6eee89c4e70a04ad674edb7a05b..2769510dd38ef914067f2eb6972f91208d88f15c 100644
--- a/ash/display/display_manager.cc
+++ b/ash/display/display_manager.cc
@@ -884,13 +884,6 @@ void DisplayManager::UpdateDisplays(
   scoped_ptr<NonDesktopDisplayUpdater> non_desktop_display_updater(
       new NonDesktopDisplayUpdater(this, delegate_));
 
-  // Do not update |displays_| if there's nothing to be updated. Without this,
-  // it will not update the display layout, which causes the bug
-  // http://crbug.com/155948.
-  if (display_changes.empty() && added_display_indices.empty() &&
-      removed_displays.empty()) {
-    return;
-  }
   // Clear focus if the display has been removed, but don't clear focus if
   // the destkop has been moved from one display to another
   // (mirror -> docked, docked -> single internal).
@@ -900,6 +893,22 @@ void DisplayManager::UpdateDisplays(
   if (delegate_)
     delegate_->PreDisplayConfigurationChange(clear_focus);
 
+  // Do not update |displays_| if there's nothing to be updated. Without this,
+  // it will not update the display layout, which causes the bug
+  // http://crbug.com/155948.
+  if (display_changes.empty() && added_display_indices.empty() &&
+      removed_displays.empty()) {
+    // When changing from software mirroring mode to sinlge display mode, it
+    // is possible there is no need to update |displays_| and we early out
+    // here. But we still want to run the PostDisplayConfigurationChange()
+    // cause there are some clients need to act on this, e.g.
+    // TouchTransformerController needs to adjust the TouchTransformer when
+    // switching from dual displays to single display.
+    if (delegate_)
+      delegate_->PostDisplayConfigurationChange();
+    return;
+  }
+
   size_t updated_index;
   if (UpdateSecondaryDisplayBoundsForLayout(&new_displays, &updated_index) &&
       std::find(added_display_indices.begin(),