allows for scrolling yoverflow (#507)

Author: vogtn

src/components/mgt-teams-channel-picker/mgt-teams-channel-picker.scss

@@ -204,6 +204,8 @@ $dropdown-item-selected-background: var(--dropdown-item-selected-background, #de
     font-weight: normal;
     color: $font-color;
     display: none;
+    max-height: 346px;
+    overflow-y: auto;
 
     &.visible {
       display: block;

src/components/mgt-teams-channel-picker/mgt-teams-channel-picker.ts

@@ -452,6 +452,12 @@ export class MgtTeamsChannelPicker extends MgtTemplatedComponent {
       selected: isSelected
     };
 
+    const dropDown = this.renderRoot.querySelector('.dropdown');
+
+    if (dropDown.children[this._focusedIndex]) {
+      dropDown.children[this._focusedIndex].scrollIntoView(false);
+    }
+
     return html`
       <div @click=${() => this.handleItemClick(itemState)} class="${classMap(classes)}">
         <div class="arrow">