Do not bump group threads on leave.

app/src/main/java/org/thoughtcrime/securesms/sms/IncomingGroupUpdateMessage.java

@@ -1,7 +1,11 @@
 package org.thoughtcrime.securesms.sms;
 
+import androidx.annotation.NonNull;
+
+import org.signal.storageservice.protos.groups.local.DecryptedGroupChange;
 import org.thoughtcrime.securesms.database.model.databaseprotos.DecryptedGroupV2Context;
 import org.thoughtcrime.securesms.mms.MessageGroupContext;
+import org.whispersystems.signalservice.api.groupsv2.DecryptedGroupUtil;
 
 import static org.whispersystems.signalservice.internal.push.SignalServiceProtos.GroupContext;
 
@@ -39,4 +43,28 @@ public final class IncomingGroupUpdateMessage extends IncomingTextMessage {
     return !groupContext.isV2Group() && groupContext.requireGroupV1Properties().isQuit();
   }
 
+  @Override
+  public boolean isJustAGroupLeave() {
+    if (isGroupV2() && isUpdate()) {
+      DecryptedGroupChange decryptedGroupChange = groupContext.requireGroupV2Properties()
+                                                              .getChange();
+
+      return changeEditorOnlyWasRemoved(decryptedGroupChange) &&
+             noChangesOtherThanDeletes(decryptedGroupChange);
+    }
+
+    return false;
+  }
+
+  protected boolean changeEditorOnlyWasRemoved(@NonNull DecryptedGroupChange decryptedGroupChange) {
+    return decryptedGroupChange.getDeleteMembersCount() == 1 &&
+           decryptedGroupChange.getDeleteMembers(0).equals(decryptedGroupChange.getEditor());
+  }
+
+  protected boolean noChangesOtherThanDeletes(@NonNull DecryptedGroupChange decryptedGroupChange) {
+    DecryptedGroupChange withoutDeletedMembers = decryptedGroupChange.toBuilder()
+                                                                     .clearDeleteMembers()
+                                                                     .build();
+    return DecryptedGroupUtil.changeIsEmpty(withoutDeletedMembers);
+  }
 }

app/src/main/java/org/thoughtcrime/securesms/sms/IncomingTextMessage.java

@@ -253,6 +253,13 @@ public class IncomingTextMessage implements Parcelable {
     return false;
   }
 
+  /**
+   * @return True iff the message is only a group leave of a single member.
+   */
+  public boolean isJustAGroupLeave() {
+    return false;
+  }
+
   public boolean isUnidentified() {
     return unidentified;
   }