From the gif above, would it be better to expand the progress to the whole window ? Like Test Explorer in VS?

Also, I feel it might be confusing to leave the tree item blank during loading.
I would expected something like this to indicate we are loading something

And one last thought (not a blocking issue for this PR, might be something in the future)
I have seen a lot of places in VS, like the FAR window

Test Explorer

When the 'tree view' item is selected, the blue selection extends to the end.

On earth they are all customized ListView instead of tree view. Since we are having more and more tree view like UI in the future I feel we might need to create our own in Roslyn and applies all of them to our current UI.

Updated the top gif to the new iteration @Cosifne