diff --git a/polygerrit-ui/app/elements/diff/gr-context-controls/gr-context-controls_test.ts b/polygerrit-ui/app/elements/diff/gr-context-controls/gr-context-controls_test.ts
new file mode 100644
index 0000000..37b8723
--- /dev/null
+++ b/polygerrit-ui/app/elements/diff/gr-context-controls/gr-context-controls_test.ts
@@ -0,0 +1,212 @@
+/**
+ * @license
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import '../../../test/common-test-setup-karma';
+import '../gr-diff/gr-diff-group';
+import './gr-context-controls';
+import {GrContextControls} from './gr-context-controls';
+import {flush} from '@polymer/polymer/lib/legacy/polymer.dom';
+
+import {GrDiffLine, GrDiffLineType} from '../gr-diff/gr-diff-line';
+import {GrDiffGroup, GrDiffGroupType} from '../gr-diff/gr-diff-group';
+import {DiffFileMetaInfo, DiffInfo} from '../../../api/diff';
+
+const blankFixture = fixtureFromElement('div');
+
+suite('gr-context-control tests', () => {
+  let element: GrContextControls;
+
+  setup(async () => {
+    element = document.createElement('gr-context-controls');
+    element.diff = ({content: []} as any) as DiffInfo;
+    element.renderPreferences = {};
+    element.section = document.createElement('div');
+    blankFixture.instantiate().appendChild(element);
+    await flush();
+  });
+
+  function createContextGroups(options: {offset?: number; count?: number}) {
+    const offset = options.offset || 0;
+    const numLines = options.count || 10;
+    const lines = [];
+    for (let i = 0; i < numLines; i++) {
+      const line = new GrDiffLine(GrDiffLineType.BOTH);
+      line.beforeNumber = offset + i + 1;
+      line.afterNumber = offset + i + 1;
+      line.text = 'lorem upsum';
+      lines.push(line);
+    }
+
+    return [new GrDiffGroup(GrDiffGroupType.BOTH, lines)];
+  }
+
+  test('no +10 buttons for 10 or less lines', async () => {
+    element.contextGroups = createContextGroups({count: 10});
+    await flush();
+
+    const buttons = element.shadowRoot!.querySelectorAll(
+      'gr-button.showContext'
+    );
+    assert.equal(buttons.length, 1);
+    assert.equal(buttons[0].textContent!.trim(), '+10 common lines');
+  });
+
+  test('context control at the top', async () => {
+    element.contextGroups = createContextGroups({offset: 0, count: 20});
+    element.showBelow = true;
+    await flush();
+
+    const buttons = element.shadowRoot!.querySelectorAll(
+      'gr-button.showContext'
+    );
+
+    assert.equal(buttons.length, 2);
+    assert.equal(buttons[0].textContent!.trim(), '+20 common lines');
+    assert.equal(buttons[1].textContent!.trim(), '+10');
+
+    assert.include([...buttons[0].classList.values()], 'belowButton');
+    assert.include([...buttons[1].classList.values()], 'belowButton');
+  });
+
+  test('context control in the middle', async () => {
+    element.contextGroups = createContextGroups({offset: 10, count: 20});
+    element.showAbove = true;
+    element.showBelow = true;
+    await flush();
+
+    const buttons = element.shadowRoot!.querySelectorAll(
+      'gr-button.showContext'
+    );
+
+    assert.equal(buttons.length, 3);
+    assert.equal(buttons[0].textContent!.trim(), '+20 common lines');
+    assert.equal(buttons[1].textContent!.trim(), '+10');
+    assert.equal(buttons[2].textContent!.trim(), '+10');
+
+    assert.include([...buttons[0].classList.values()], 'centeredButton');
+    assert.include([...buttons[1].classList.values()], 'aboveButton');
+    assert.include([...buttons[2].classList.values()], 'belowButton');
+  });
+
+  test('context control at the bottom', async () => {
+    element.contextGroups = createContextGroups({offset: 30, count: 20});
+    element.showAbove = true;
+    await flush();
+
+    const buttons = element.shadowRoot!.querySelectorAll(
+      'gr-button.showContext'
+    );
+
+    assert.equal(buttons.length, 2);
+    assert.equal(buttons[0].textContent!.trim(), '+20 common lines');
+    assert.equal(buttons[1].textContent!.trim(), '+10');
+
+    assert.include([...buttons[0].classList.values()], 'aboveButton');
+    assert.include([...buttons[1].classList.values()], 'aboveButton');
+  });
+
+  test('context control with block expansion at the top', async () => {
+    element.renderPreferences!.use_block_expansion = true;
+    element.diff!.meta_b = ({
+      syntax_tree: [],
+    } as any) as DiffFileMetaInfo;
+    element.contextGroups = createContextGroups({offset: 0, count: 20});
+    element.showBelow = true;
+    await flush();
+
+    const fullExpansionButtons = element.shadowRoot!.querySelectorAll(
+      '.fullExpansion gr-button'
+    );
+    const partialExpansionButtons = element.shadowRoot!.querySelectorAll(
+      '.partialExpansion gr-button'
+    );
+    const blockExpansionButtons = element.shadowRoot!.querySelectorAll(
+      '.blockExpansion gr-button'
+    );
+    assert.equal(fullExpansionButtons.length, 1);
+    assert.equal(partialExpansionButtons.length, 1);
+    assert.equal(blockExpansionButtons.length, 1);
+    assert.equal(blockExpansionButtons[0].textContent!.trim(), '+Block');
+    assert.include(
+      [...blockExpansionButtons[0].classList.values()],
+      'belowButton'
+    );
+  });
+
+  test('context control with block expansion in the middle', async () => {
+    element.renderPreferences!.use_block_expansion = true;
+    element.diff!.meta_b = ({
+      syntax_tree: [],
+    } as any) as DiffFileMetaInfo;
+
+    element.contextGroups = createContextGroups({offset: 10, count: 20});
+    element.showAbove = true;
+    element.showBelow = true;
+    await flush();
+
+    const fullExpansionButtons = element.shadowRoot!.querySelectorAll(
+      '.fullExpansion gr-button'
+    );
+    const partialExpansionButtons = element.shadowRoot!.querySelectorAll(
+      '.partialExpansion gr-button'
+    );
+    const blockExpansionButtons = element.shadowRoot!.querySelectorAll(
+      '.blockExpansion gr-button'
+    );
+    assert.equal(fullExpansionButtons.length, 1);
+    assert.equal(partialExpansionButtons.length, 2);
+    assert.equal(blockExpansionButtons.length, 2);
+    assert.equal(blockExpansionButtons[0].textContent!.trim(), '+Block');
+    assert.equal(blockExpansionButtons[1].textContent!.trim(), '+Block');
+    assert.include(
+      [...blockExpansionButtons[0].classList.values()],
+      'aboveButton'
+    );
+    assert.include(
+      [...blockExpansionButtons[1].classList.values()],
+      'belowButton'
+    );
+  });
+
+  test('context control with block expansion at the bottom', async () => {
+    element.renderPreferences!.use_block_expansion = true;
+    element.diff!.meta_b = ({
+      syntax_tree: [],
+    } as any) as DiffFileMetaInfo;
+    element.contextGroups = createContextGroups({offset: 30, count: 20});
+    element.showAbove = true;
+    await flush();
+
+    const fullExpansionButtons = element.shadowRoot!.querySelectorAll(
+      '.fullExpansion gr-button'
+    );
+    const partialExpansionButtons = element.shadowRoot!.querySelectorAll(
+      '.partialExpansion gr-button'
+    );
+    const blockExpansionButtons = element.shadowRoot!.querySelectorAll(
+      '.blockExpansion gr-button'
+    );
+    assert.equal(fullExpansionButtons.length, 1);
+    assert.equal(partialExpansionButtons.length, 1);
+    assert.equal(blockExpansionButtons.length, 1);
+    assert.equal(blockExpansionButtons[0].textContent!.trim(), '+Block');
+    assert.include(
+      [...blockExpansionButtons[0].classList.values()],
+      'aboveButton'
+    );
+  });
+});