Merge tabs of multi-instance when display is changed.

On partner foldable phones, when the secondary display is off, there
is only one targetable displays and only #onDisplayChanged is called.
This CL tries to merge tabs #onDisplayChanged as well.

Bug: 1088202
Change-Id: Id21960b40bceb4b04fce7a2718c0e33bee0f4920
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2272131Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Lijin Shen <lazzzis@google.com>
Cr-Commit-Position: refs/heads/master@{#786055}

chrome/android/java/src/org/chromium/chrome/browser/multiwindow/MultiInstanceManager.java

@@ -20,6 +20,7 @@ import androidx.annotation.Nullable;
 import androidx.annotation.VisibleForTesting;
 
 import org.chromium.base.ActivityState;
+import org.chromium.base.ApiCompatibilityUtils;
 import org.chromium.base.ApplicationStatus;
 import org.chromium.base.CommandLine;
 import org.chromium.base.metrics.RecordUserAction;
@@ -85,6 +86,7 @@ public class MultiInstanceManager
     private boolean mShouldMergeOnConfigurationChange;
     private boolean mIsRecreating;
     private int mDisplayId;
+    private static List<Integer> sTestDisplayIds;
 
     /**
      * Create a new {@link MultiInstanceManager}.
@@ -220,7 +222,13 @@ public class MultiInstanceManager
 
             @Override
             public void onDisplayChanged(int displayId) {
-                // TODO(crbug.com/824954): try to merge tabs sharing logic w/ onDisplayRemoved
+                if (displayId == mDisplayId) return;
+                List<Integer> ids = sTestDisplayIds != null
+                        ? sTestDisplayIds
+                        : ApiCompatibilityUtils.getTargetableDisplayIds(mActivity);
+                if (ids.size() == 1 && ids.get(0).equals(mDisplayId)) {
+                    maybeMergeTabs();
+                }
             }
         };
         displayManager.registerDisplayListener(mDisplayListener, null);
@@ -433,4 +441,9 @@ public class MultiInstanceManager
     public DisplayManager.DisplayListener getDisplayListenerForTesting() {
         return mDisplayListener;
     }
+
+    @VisibleForTesting
+    public static void setTestDisplayIds(List<Integer> testDisplayIds) {
+        sTestDisplayIds = testDisplayIds;
+    }
 }

chrome/android/javatests/src/org/chromium/chrome/browser/tabmodel/TabModelMergingTest.java

@@ -54,6 +54,7 @@ import org.chromium.content_public.browser.test.util.CriteriaHelper;
 import org.chromium.content_public.browser.test.util.TestThreadUtils;
 import org.chromium.ui.test.util.UiRestriction;
 
+import java.util.Collections;
 import java.util.concurrent.TimeoutException;
 
 /**
@@ -559,4 +560,49 @@ public class TabModelMergingTest {
         mActivity1.finishAndRemoveTask();
         mActivity2.finishAndRemoveTask();
     }
+
+    @Test
+    @LargeTest
+    @EnableFeatures(ChromeFeatureList.ANDROID_MULTIPLE_DISPLAY)
+    public void testMergeOnMultiDisplay_OnDisplayChanged() throws TimeoutException {
+        TestThreadUtils.runOnUiThreadBlocking(() -> {
+            mActivity1.saveState();
+            mActivity2.saveState();
+        });
+        MultiInstanceManager m1 = mActivity1.getMultiInstanceMangerForTesting();
+        MultiInstanceManager m2 = mActivity2.getMultiInstanceMangerForTesting();
+
+        // Ensure Activity 1 is resumed on the front.
+        Intent intent = new Intent(mActivity1, mActivity1.getClass());
+        intent.addFlags(Intent.FLAG_ACTIVITY_REORDER_TO_FRONT);
+        mActivity1.startActivity(intent);
+        waitForActivityStateChange(ActivityState.RESUMED, mActivity2, false);
+        waitForActivityStateChange(ActivityState.RESUMED, mActivity1, true);
+
+        MultiInstanceManager.setTestDisplayIds(Collections.singletonList(0));
+        m1.setCurrentDisplayIdForTesting(0);
+        m2.setCurrentDisplayIdForTesting(1);
+
+        TestThreadUtils.runOnUiThreadBlocking(() -> {
+            m1.getDisplayListenerForTesting().onDisplayChanged(1);
+            m2.getDisplayListenerForTesting().onDisplayChanged(1);
+        });
+
+        CriteriaHelper.pollUiThread(new Criteria("Total tab count incorrect.") {
+            @Override
+            public boolean isSatisfied() {
+                return mActivity1.getTabModelSelector().getTotalTabCount() == 7;
+            }
+        });
+        CriteriaHelper.pollUiThread(new Criteria("CTA2 should be destroyed"
+                + "CTA state: " + mActivity1State + " - CTA2State: " + mActivity2State) {
+            @Override
+            public boolean isSatisfied() {
+                return mActivity2State == ActivityState.DESTROYED
+                        && mActivity1State != ActivityState.DESTROYED;
+            }
+        });
+        mActivity1.finishAndRemoveTask();
+        mActivity2.finishAndRemoveTask();
+    }
 }