CrOS Shelf: Fix a crash when the display is extremely narrow

The crash happens when the display width is less than about 210 pixels,
and one clicks on the overflow button.

Add a test to prevent future regressions.

Bug: 977043
Change-Id: I8d1fa02eee829bf4ca0d45000b66842dcd2e65a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1808378Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697218}

ash/shelf/shelf_view.cc

@@ -1233,8 +1233,10 @@ ShelfView::AppCenteringStrategy ShelfView::CalculateAppCenteringStrategy() {
       last_visible_index_++;
     } else {
       strategy.overflow = true;
-      // Make space for the overflow button by showing one fewer app icon.
-      last_visible_index_--;
+      // Make space for the overflow button by showing one fewer app icon. If
+      // we already don't have enough space, don't decrement the last visible
+      // index further than -1.
+      last_visible_index_ = std::max(-1, last_visible_index_ - 1);
       break;
     }
   }

ash/shelf/shelf_view.h

@@ -575,7 +575,8 @@ class ASH_EXPORT ShelfView : public views::AccessiblePaneView,
   int first_visible_index_ = -1;
 
   // Last index of an app launcher button that is visible (does not go into
-  // overflow), or -1 if there are no apps.
+  // overflow), or -1 if there are no apps (or if only the overflow button is
+  // visible).
   int last_visible_index_ = -1;
 
   std::unique_ptr<views::BoundsAnimator> bounds_animator_;

ash/shelf/shelf_view_unittest.cc

@@ -2049,6 +2049,27 @@ TEST_F(ShelfViewTest, TestDragWithinOverflow) {
   test_api_->HideOverflowBubble();
 }
 
+// Checks how the overflow button and menu get laid out when the display is
+// very narrow.
+TEST_F(ShelfViewTest, TestOverflowWithNarrowDisplay) {
+  // No overflow bubble when scrollable shelf enabled.
+  // TODO(https://crbug.com/1002576): revisit when scrollable shelf is launched.
+  if (chromeos::switches::ShouldShowScrollableShelf())
+    return;
+
+  UpdateDisplay("200x600");
+
+  AddAppShortcutsUntilOverflow();
+  OverflowButton* overflow_button = shelf_view_->GetOverflowButton();
+  EXPECT_TRUE(overflow_button->GetVisible());
+
+  ui::test::EventGenerator* generator = GetEventGenerator();
+  generator->set_current_screen_location(
+      overflow_button->GetBoundsInScreen().CenterPoint());
+  generator->ClickLeftButton();
+  ASSERT_TRUE(shelf_view_->IsShowingOverflowBubble());
+}
+
 // Checks creating app shortcut for an opened platform app in overflow bubble
 // should be invisible to the shelf. See crbug.com/605793.
 TEST_F(ShelfViewTest, CheckOverflowStatusPinOpenedAppToShelf) {