[Tab Groups Collapse] Shift the active tab when collapsing the active tab.

Bug: 1018230
Change-Id: I837514789ae6809c3d2eb8f3927837c48c416eb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209236
Commit-Queue: Charlene Yan <cyan@chromium.org>
Reviewed-by: Taylor Bergquist <tbergquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773819}

chrome/browser/ui/views/tabs/browser_tab_strip_controller.cc

@@ -349,8 +349,20 @@ void BrowserTabStripController::MoveGroup(const tab_groups::TabGroupId& group,
 
 void BrowserTabStripController::ToggleTabGroupCollapsedState(
     const tab_groups::TabGroupId group) {
+  // If the active tab is in the group that is toggling to collapse, the active
+  // tab should switch to the next available tab. If there are no available tabs
+  // for the active tab to switch to, the group will not toggle to collapse.
+  const bool is_currently_collapsed = IsGroupCollapsed(group);
+  if (!is_currently_collapsed) {
+    const base::Optional<int> next_active =
+        FindNextAvailableActiveTabForCollapsingGroup(group);
+    if (!next_active.has_value())
+      return;
+    model_->ActivateTabAt(next_active.value(),
+                          {TabStripModel::GestureType::kOther});
+  }
   tab_groups::TabGroupVisualData new_data(
-      GetGroupTitle(group), GetGroupColorId(group), !IsGroupCollapsed(group));
+      GetGroupTitle(group), GetGroupColorId(group), !is_currently_collapsed);
   model_->group_model()->GetTabGroup(group)->SetVisualData(new_data, true);
 }
 
@@ -692,3 +704,29 @@ void BrowserTabStripController::UpdateStackedLayout() {
   tabstrip_->set_adjust_layout(adjust_layout);
   tabstrip_->SetStackedLayout(stacked_layout);
 }
+
+base::Optional<int>
+BrowserTabStripController::FindNextAvailableActiveTabForCollapsingGroup(
+    const tab_groups::TabGroupId group) const {
+  const int starting_index = GetActiveIndex();
+  const std::vector<int> tabs_in_group = ListTabsInGroup(group);
+
+  if (starting_index < tabs_in_group.front() ||
+      starting_index > tabs_in_group.back()) {
+    return starting_index;
+  }
+
+  for (int i = 0; i < model_->count(); i++) {
+    int current_index = (i + starting_index) % model_->count();
+    base::Optional<tab_groups::TabGroupId> current_group =
+        model_->GetTabGroupForTab(current_index);
+    if (!current_group.has_value() ||
+        (!IsGroupCollapsed(current_group.value()) && current_group != group)) {
+      return current_index;
+    }
+    const std::vector<int> tabs_in_group =
+        ListTabsInGroup(current_group.value());
+  }
+
+  return base::nullopt;
+}

chrome/browser/ui/views/tabs/browser_tab_strip_controller.h

@@ -149,6 +149,11 @@ class BrowserTabStripController : public TabStripController,
   // Resets the tabstrips stacked layout (true or false) from prefs.
   void UpdateStackedLayout();
 
+  // Finds the next available tab to switch to as the active tab when the
+  // |index| is collapsing. Returns base::nullopt if there are no valid tabs.
+  base::Optional<int> FindNextAvailableActiveTabForCollapsingGroup(
+      const tab_groups::TabGroupId group) const;
+
   TabStripModel* model_;
 
   TabStrip* tabstrip_;