Fix panel transition abruptly snapping to the end

packages/vue-components/src/__tests__/Panels.spec.js

@@ -60,6 +60,26 @@ describe('NestedPanels', () => {
     expect(wrapper.element).toMatchSnapshot();
   });
 
+  test('transition height is correctly calculated', async () => {
+    const wrapper = mount(NestedPanel, {
+      slots: {
+        header: 'test header',
+        default: 'Some panel content\nsome other text',
+      },
+      propsData: {
+        preload: true,
+      },
+      attachTo: document.body,
+    });
+    const panelElement = wrapper.element.querySelector('.card-collapse');
+    Object.defineProperty(panelElement, 'scrollHeight', { configurable: true, value: 10 });
+    const bottomSwitch = wrapper.element.querySelector('.card-body > .collapse-button');
+    bottomSwitch.style.marginBottom = '13px';
+    // click on header
+    await wrapper.find('div.card-header').trigger('click');
+    expect(wrapper.element).toMatchSnapshot();
+  });
+
   test('should have span.anchor when id is present', async () => {
     const wrapper = mount(NestedPanel, {
       propsData: {

packages/vue-components/src/__tests__/__snapshots__/Panels.spec.js.snap

@@ -432,3 +432,100 @@ exports[`NestedPanels should show header when collapsed with expandHeaderless as
   </div>
 </div>
 `;
+
+exports[`NestedPanels transition height is correctly calculated 1`] = `
+<div
+  class="card-container"
+>
+  <!---->
+
+  <div
+    class="card expandable-card"
+  >
+    <div
+      class="card-header header-toggle bg-light"
+    >
+      <div
+        class="caret-wrapper"
+      >
+        <!---->
+      </div>
+
+      <div
+        class="header-wrapper card-title bg-light"
+      >
+        test header
+      </div>
+
+      <div
+        class="button-wrapper"
+      >
+        <button
+          class="collapse-button btn btn-outline-secondary"
+          type="button"
+        >
+          <span
+            aria-hidden="true"
+            class="collapse-icon glyphicon glyphicon-menu-down opened"
+          />
+        </button>
+
+        <button
+          class="close-button btn btn-outline-secondary"
+          type="button"
+        >
+          <span
+            aria-hidden="true"
+            class="glyphicon glyphicon-remove"
+          />
+        </button>
+
+        <button
+          class="popup-button btn btn-outline-secondary"
+          style="display: none;"
+          type="button"
+        >
+          <span
+            aria-hidden="true"
+            class="glyphicon glyphicon-new-window"
+          />
+        </button>
+      </div>
+    </div>
+
+    <div
+      class="card-collapse"
+      style="max-height: 23px;"
+    >
+      <div
+        class="card-body"
+      >
+        Some panel content
+some other text 
+        <!---->
+
+        <button
+          class="collapse-button btn btn-outline-secondary"
+          style="margin-bottom: 13px;"
+          type="button"
+        >
+          <span
+            aria-hidden="true"
+            class="collapse-icon glyphicon glyphicon-menu-down opened"
+          />
+        </button>
+      </div>
+
+      <hr
+        style="display: none;"
+      />
+    </div>
+
+    <transition-stub
+      name="peek-read-more-fade"
+    >
+      <!---->
+    </transition-stub>
+  </div>
+</div>
+`;

packages/vue-components/src/panels/PanelBase.js

@@ -150,7 +150,7 @@ export default {
       } else {
         // Expand panel
         this.$refs.panel.style.transition = 'max-height 0.5s ease-in-out';
-        this.$refs.panel.style.maxHeight = `${this.$refs.panel.scrollHeight}px`;
+        this.$refs.panel.style.maxHeight = `${this.getMaxHeight()}px`;
       }
 
       this.localExpanded = !this.localExpanded;
@@ -174,7 +174,7 @@ export default {
           DOM update (nextTick) before setting maxHeight for transition.
         */
         this.$nextTick(() => {
-          this.$refs.panel.style.maxHeight = `${this.$refs.panel.scrollHeight}px`;
+          this.$refs.panel.style.maxHeight = `${this.getMaxHeight()}px`;
         });
       });
     },
@@ -196,7 +196,28 @@ export default {
       }
 
       // For expansion transition to 'continue' after src is loaded.
-      this.$refs.panel.style.maxHeight = `${this.$refs.panel.scrollHeight}px`;
+      this.$refs.panel.style.maxHeight = `${this.getMaxHeight()}px`;
+    },
+    getMaxHeight() {
+      if (!this.bottomSwitchBool) {
+        return this.$refs.panel.scrollHeight;
+      }
+      /*
+        Collapse button at bottom of panel's bottom margin is not included in panel's scrollHeight.
+
+        It's bottom margin is added to the maxHeight of the panel to enable a smooth transition.
+        Otherwise, there would be an instant transition when reaching the end of the panel content.
+       */
+      const bottomSwitch = document.querySelector('.card-body > .collapse-button');
+      if (bottomSwitch == null) {
+        return this.$refs.panel.scrollHeight;
+      }
+      const bottomSwitchStyle = window.getComputedStyle(bottomSwitch);
+      const bottomSwitchBottomMargin = parseFloat(bottomSwitchStyle.marginBottom);
+      if (Number.isNaN(bottomSwitchBottomMargin)) {
+        return this.$refs.panel.scrollHeight;
+      }
+      return this.$refs.panel.scrollHeight + bottomSwitchBottomMargin;
     },
     initPanel() {
       this.$refs.panel.addEventListener('transitionend', (event) => {