"Select a source" message appears when source is already selected

[eloquence]

Reported by @creviera in #1275 (comment)

Steps to reproduce

Select a source message appears when a source is already selected.

1. Select a source
2. Delete files and messages
3. Log out
4. Log back in

Expected behavior

You continue to see the tear pattern in the conversation view of the selected source

Actual behavior

The "Select a source" message is displayed on top of the tear pattern until the next sync

[conorsch]

Was able to reproduce this one today. Didn't grab a screenshot, because the sync clears up the artifact, as noted above.

[gonzalo-bulnes]

I was able to see a flash of content, I suppose that counts as a reproduction. 👍
@creviera I'm interested in learning to slow down the sync rate, so that we can reproduce this more reliably. 🙂 Not a blocker for now, though.

[sssoleileraaa]

Tip for debugging: this happens if you log in while a source who's files and messages have been deleted is already selected. So no need to keep deleting files and messages. You can do it just once.

[gonzalo-bulnes]

Note: the sync doesn't seem to be involved.

[sssoleileraaa]

I think the issue might be between these two calls clobbering each other:

securedrop-client/securedrop_client/logic.py

Lines 607 to 608 in 5fdcd71

	self.update_sources()
	self.gui.refresh_current_source_conversation()

Here is where we decided whether or not to display the "Select a source" message:

securedrop-client/securedrop_client/gui/widgets.py

Line 612 in 5fdcd71

self.empty_conversation_view.show_no_source_selected_message()

and the update_deletion_markers is where we decide whether or not to display the "Files and messages deleted" message.

These are the two messages showing up on top of each other.

[sssoleileraaa]

In case it helps...