Ensure that `Owned Files` is not switchable when no files are owned

When plugin registers itself for tab display even if `nothing` is
returned Gerrit still shows the pointer and allows clicking at it,
redirecting to the empty tab when done.

Traverse through the shadowRoots down to the tab and disable/enable
it when needed. As a result it is no longer possible to select an empty
tab without content.

Change-Id: Ib9da5fe0ed8ba564f8d62de2f8b13179ae232a4e

owners/web/gr-owned-files.ts

diff --git a/owners/web/gr-owned-files.ts b/owners/web/gr-owned-files.ts
index 5f5d919..56274d4 100644
--- a/owners/web/gr-owned-files.ts
+++ b/owners/web/gr-owned-files.ts
@@ -71,11 +71,36 @@
 @customElement(OWNED_FILES_TAB_HEADER)
 export class OwnedFilesTabHeader extends OwnedFilesCommon {
   static override get styles() {
-    return [...OwnedFilesCommon.commonStyles()];
+    return [
+      ...OwnedFilesCommon.commonStyles(),
+      css`
+        [hidden] {
+          display: none;
+        }
+      `,
+    ];
   }
 
   override render() {
-    if (this.hidden) return nothing;
+    // even if `nothing` is returned Gerrit still shows the pointer and allows
+    // clicking at it, redirecting to the empty tab when done; traverse through
+    // the shadowRoots down to the tab and disable/enable it when needed
+    const tabParent = document
+      .querySelector('#pg-app')
+      ?.shadowRoot?.querySelector('#app-element')
+      ?.shadowRoot?.querySelector('main > gr-change-view')
+      ?.shadowRoot?.querySelector(
+        '#tabs > paper-tab[data-name="change-view-tab-header-owners"]'
+      );
+    if (this.hidden) {
+      if (tabParent && !tabParent.getAttribute('disabled')) {
+        tabParent.setAttribute('disabled', 'disabled');
+      }
+      return nothing;
+    }
+    if (tabParent && tabParent.getAttribute('disabled')) {
+      tabParent.removeAttribute('disabled');
+    }
     return html`<div>Owned Files</div>`;
   }
 }