Custom modal aux click (#6891)

## Motivation for features / changes
On some browsers, the [custom modal
close](https://github.com/tensorflow/tensorboard/blob/0627ad5480b309a5f3f69f39e52670496c2e5863/tensorboard/webapp/widgets/custom_modal/custom_modal.ts#L116)
logic will be triggered immediately after releasing the right click
button, which causes the modal to be closed as soon as it is opened.

## Technical description of changes
The modal close event handler will ignore the `auxclick` (right click)
mouseup event. Modals can still be closed with left clicks and the
`contextmenu` event.

## Screenshots of UI changes (or N/A)
Status quo:
![image](https://github.com/user-attachments/assets/5b4c0c8f-4878-4cbe-a46a-cd425c188061)

Author: hoonji

tensorboard/webapp/widgets/custom_modal/custom_modal.ts

@@ -109,23 +109,23 @@ export class CustomModal {
     const customModalRef = new CustomModalRef(overlayRef);
     this.customModalRefs.push(customModalRef);
 
-    // setTimeout to prevent closing immediately after modal open.
-    setTimeout(() => {
-      const outsidePointerEventsSubscription = overlayRef
-        .outsidePointerEvents()
-        .subscribe((event) => {
-          // Only close when click is outside of every modal
-          if (
-            this.customModalRefs.every(
-              (ref) =>
-                !isMouseEventInElement(event, ref.overlayRef.overlayElement)
-            )
-          ) {
-            this.closeAll();
-          }
-        });
-      customModalRef.subscriptions.push(outsidePointerEventsSubscription);
-    });
+    const outsidePointerEventsSubscription = overlayRef
+      .outsidePointerEvents()
+      .subscribe((event) => {
+        // Prevent the right click mouseup event from immediately closing the modal.
+        if (event.type === 'auxclick') return;
+
+        // Only close when click is outside of every modal
+        if (
+          this.customModalRefs.every(
+            (ref) =>
+              !isMouseEventInElement(event, ref.overlayRef.overlayElement)
+          )
+        ) {
+          this.closeAll();
+        }
+      });
+    customModalRef.subscriptions.push(outsidePointerEventsSubscription);
 
     const keydownEventsSubscription = overlayRef
       .keydownEvents()