Steps to reproduce:
1. Create a page of type "Panel", select a subset of apps (for example blogs, calendar and wiki).
2. Visit the page, you'll see the apps in the right menu.
3. Then go to edit the page.
In Step 1, you will see the tree view directly.
In Step 3, the app tree can reflect the previously added apps, if you want to add one more app, you only need to select the new one you want to add, or if you want to remove one, you just need to select the previously added apps you want to remove.
In Step 1, you can't see the tree view unless you check the application checkbox first.
In Step 3, the app tree does not reflect the previously added apps, so if you wanted to add one more app, you need to select all previous ones plus the new one, or if you wanted to remove one, you need to select all previously added apps minus the one you want to remove.