commit d6a47665b81b0177be849b690601e9b726bf44c4
Author: Ben Rohlfs <brohlfs@google.com>
Date:   Thu Oct 1 14:42:47 2020 +0200

    Fix "Do not update attention set" message
    
    If we are just removing users from the attention set, then we should
    not show "do not update", but "do not add" instead.
    
    Change-Id: I17f199a998f82d36a011147eda77a11c4f5919b4

diff --git a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts
index 95d76d8..d774607 100644
--- a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts
+++ b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts
@@ -97,7 +97,11 @@ import {
   PolymerSplice,
   PolymerSpliceChange,
 } from '@polymer/polymer/interfaces';
-import {assertNever} from '../../../utils/common-util';
+import {
+  areSetsEqual,
+  assertNever,
+  containsAll,
+} from '../../../utils/common-util';
 import {CommentThread, isDraft} from '../../diff/gr-comment-api/gr-comment-api';
 import {GrTextarea} from '../../shared/gr-textarea/gr-textarea';
 import {GrAccountChip} from '../../shared/gr-account-chip/gr-account-chip';
@@ -1065,6 +1069,23 @@ export class GrReplyDialog extends KeyboardShortcutMixin(
     );
   }
 
+  _computeDoNotUpdateMessage(
+    currentAttentionSet?: Set<AccountId>,
+    newAttentionSet?: Set<AccountId>
+  ) {
+    if (!currentAttentionSet || !newAttentionSet) return '';
+    if (areSetsEqual(currentAttentionSet, newAttentionSet)) {
+      return 'No changes to the attention set.';
+    }
+    if (containsAll(currentAttentionSet, newAttentionSet)) {
+      return 'No additions to the attention set.';
+    }
+    console.error(
+      '_computeDoNotUpdateMessage() should not be called when users were added to the attention set.'
+    );
+    return '';
+  }
+
   _computeNewAttentionAccounts(
     _?: ServerInfo,
     currentAttentionSet?: Set<AccountId>,
diff --git a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog_html.ts b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog_html.ts
index 247e891..f076981 100644
--- a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog_html.ts
+++ b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog_html.ts
@@ -371,7 +371,10 @@ export const htmlTemplate = html`
               is="dom-if"
               if="[[_isNewAttentionEmpty(serverConfig, _currentAttentionSet, _newAttentionSet)]]"
             >
-              <span>Do not update the attention set.</span>
+              <span
+                >[[_computeDoNotUpdateMessage(_currentAttentionSet,
+                _newAttentionSet)]]</span
+              >
             </template>
             <template
               is="dom-if"
diff --git a/polygerrit-ui/app/utils/common-util.ts b/polygerrit-ui/app/utils/common-util.ts
index d144272..183e56c 100644
--- a/polygerrit-ui/app/utils/common-util.ts
+++ b/polygerrit-ui/app/utils/common-util.ts
@@ -44,3 +44,25 @@ export function assertNever(obj: never, msg: string): never {
   console.error(msg, obj);
   throw new Error(msg);
 }
+
+/**
+ * Returns true, if both sets contain the same members.
+ */
+export function areSetsEqual<T>(a: Set<T>, b: Set<T>): boolean {
+  if (a.size !== b.size) {
+    return false;
+  }
+  return containsAll(a, b);
+}
+
+/**
+ * Returns true, if 'set' contains 'subset'.
+ */
+export function containsAll<T>(set: Set<T>, subSet: Set<T>): boolean {
+  for (const value of subSet) {
+    if (!set.has(value)) {
+      return false;
+    }
+  }
+  return true;
+}
diff --git a/polygerrit-ui/app/utils/common-util_test.js b/polygerrit-ui/app/utils/common-util_test.js
index 60c0b0a..917d652 100644
--- a/polygerrit-ui/app/utils/common-util_test.js
+++ b/polygerrit-ui/app/utils/common-util_test.js
@@ -16,7 +16,7 @@
  */
 
 import '../test/common-test-setup-karma.js';
-import {hasOwnProperty} from './common-util.js';
+import {hasOwnProperty, areSetsEqual, containsAll} from './common-util.js';
 
 suite('common-util tests', () => {
   suite('hasOwnProperty', () => {
@@ -41,4 +41,29 @@ suite('common-util tests', () => {
       assert.isFalse(hasOwnProperty(obj, 'def'));
     });
   });
+
+  test('areSetsEqual', () => {
+    assert.isTrue(areSetsEqual(new Set(), new Set()));
+    assert.isTrue(areSetsEqual(new Set([1]), new Set([1])));
+    assert.isTrue(areSetsEqual(new Set([1, 1, 1, 1]), new Set([1])));
+    assert.isTrue(areSetsEqual(new Set([1, 1, 2, 2]), new Set([2, 1, 2, 1])));
+    assert.isTrue(areSetsEqual(new Set([1, 2, 3, 4]), new Set([4, 3, 2, 1])));
+    assert.isFalse(areSetsEqual(new Set(), new Set([1])));
+    assert.isFalse(areSetsEqual(new Set([1]), new Set([2])));
+    assert.isFalse(areSetsEqual(new Set([1, 2, 4]), new Set([1, 2, 3])));
+  });
+
+  test('containsAll', () => {
+    assert.isTrue(containsAll(new Set(), new Set()));
+    assert.isTrue(containsAll(new Set([1]), new Set()));
+    assert.isTrue(containsAll(new Set([1]), new Set([1])));
+    assert.isTrue(containsAll(new Set([1, 2]), new Set([1])));
+    assert.isTrue(containsAll(new Set([1, 2]), new Set([2])));
+    assert.isTrue(containsAll(new Set([1, 2, 3, 4]), new Set([1, 4])));
+    assert.isTrue(containsAll(new Set([1, 2, 3, 4]), new Set([1, 2, 3, 4])));
+    assert.isFalse(containsAll(new Set(), new Set([2])));
+    assert.isFalse(containsAll(new Set([1]), new Set([2])));
+    assert.isFalse(containsAll(new Set([1, 2, 3, 4]), new Set([5])));
+    assert.isFalse(containsAll(new Set([1, 2, 3, 4]), new Set([1, 2, 3, 5])));
+  });
 });