Currently, with a Page open, if Resources are deleted from a Resource Dictionary, they disappear from the Resources Panel Dictionaries as expected.
This deletion successfully takes place in both the viewed Page, as well as if navigating to the open tab of the ResourceDictionary that had its Resources removed.

It is then possible to navigate to the tab of the ResourcesDictionary, and press 'Undo' to revert their deletion.
In this case, the Resources successfully come back into the ResourceDictionary, and are visible on both the Preview Stage, as well as in the Resources Panel.

However, if navigating back to the Page where the deletion originally was completed from, the Resources that were 'un-deleted' continue to be missing from the Resources Panel utnil the Page is restarted.

See attached video.

Repro Steps:

1) Create a Studio Project containing a MainPage.xaml, and introduce a ContentControl Element to its root Grid.

2) Right-Click on the ContentControl Element, and create a couple ContentControl, and ContentControl Content Templates, all stored in GlobalResources.xaml
Creating this new Resources will open up a background tab for GlobalResources.xaml

3) Still whilst remaining within MainPage.xaml, navigate to the 'Resources' Panel, select GlobalResources.xaml, and delete all of the Resources created in Step 2).

4) Navigate to the the GlobalResources.xaml tab at the top of the screen, and within the file, press Ctrl+Z to 'undo' the deletion of the Resources from this file that was done in Step 3).
Observe how in the Resources Tab, under the GlobalResources.xaml file, the Resources have been 'un-deleted'.

5) Save all open documents.

6) Navigate back to MainPage.xaml, and under the Resources Panel, select the GlobalResources.xaml entry.

Observe how when viewed through the Page, the 'un-deleted' Resources are still missing.

7) Right-Click on the ContentControl Template on-Stage or in the Navigator.

Observe how despite there being compatible Resources available in GlobalResources.xaml, the Templates are not available or visible in the Template Selector for the Element.

8) Close and re-open MainPage.xaml, then repeat Steps 6) and 7)

Observe how the Template Resources are now available.

Expected Results: When Resources are available in accessible Resource Dictionaries, they are also immediately available in all other open documents.