Honestly, this is expected. Most apps would do that.

Stealing focus is never expected and is always bad. The only reason most apps do it is because it's not actually an easy problem to solve. For this particular case it's about a text field focus, so... how do you determine whether the user is currently typing into the text field? How do you ensure that the text field remains focused? It may be inevitable that events temporarily remove the focus, so what you'd have to do is ensure it gets restored before keyboard event handling continues.

I think this is a good thing to fix, eventually.

@CelticMinstrel why not just change underlying value? Aren't these things orthogonal: UI focus and data?
I mean, you can chat in e.g. Facebook or an IM messenger. And no matter which notifications, messages, previews or pictures you get, your focus just remains where it was. The program doesn't have to change focus in order to change data right?

@Vultraz can you please give at least one well-known example of anything that would remotely alter focus on your client? Any web interface, or native, or anything.

Yeah, it shouldn't have to, so if it can be fixed in a way that doesn't remove the focus in the first place, that's even better.

Also happens when someone joins or leaves game.

This fixes it, but makes it so it's not possible to use up arrow / down arrow to scroll the sides list. It also makes it so the chat box is focused when that dialog is entered.

diff --git a/src/gui/dialogs/multiplayer/mp_join_game.cpp b/src/gui/dialogs/multiplayer/mp_join_game.cpp
index 760f9b9eb23..e36c296ff97 100644
--- a/src/gui/dialogs/multiplayer/mp_join_game.cpp
+++ b/src/gui/dialogs/multiplayer/mp_join_game.cpp
@@ -339,7 +339,7 @@ void mp_join_game::generate_side_list(window& window)
 
 	tree_view& tree = find_widget<tree_view>(&window, "side_list", false);
 
-	window.keyboard_capture(&tree);
+	window.keyboard_capture(&find_widget<chatbox>(&window, "chat", false));
 
 	tree.clear();
 	team_tree_map_.clear();

I think that's pretty good. I am curious of course if we can insert some additional easy (yet maintainable) hack to detect up/down keystrokes and do the appropriate action, but even in the proposed form, I think it's worth it.

I would merge.

Fixed with #5252.