Make replacing_by emit slightly earlier

[Rindbee]

This mainly fixes bugs that occur when replacing scene roots with certain node types.

To reproduce the bug:

1. Create a new scene,
2. Add some visible 2D child nodes to the scene,
3. Select the scene root node, right-click to open the menu, select Change Type, and then select the Camera2D type to change.
4. The display of nodes in the 2D screen is offset.

248e5bf	This PR

Fix #73009,

[ajreckof]

This is introducing a compatibility breakage. Codes that would be broken by this change can most likely just await tree_entered on the node sent with the signal. Another solution would be to defer the connection to the signal which would put the call after children have been moved.

[ajreckof]

To be clear even with the compatibility breakage I'm fully in favor of this being merged.

[KoBeWi]

I assume the problem is set_edited_scene() connected to replacing_by signal? Wouldn't it be enough to manually call set_edited_scene() when replacing root?
Also looks like the PR needs rebase, because it removes a file that doesn't seem to exist.

[Rindbee]

Some classes require us to check the current scene when entering the tree in order to improve the interactivity of the editor.

As for the situation when the node exits the tree, I forgot whether I have investigated it.

If no class needs to check the current scene when exiting, it is OK to manually call set_edited_scene() before replace_by(). However, in this case, it may be necessary to check whether the replaced node is the current scene root.

[KoBeWi]

it may be necessary to check whether the replaced node is the current scene root.

That's what I mean. Whenever a node is being replaced (by scene tree dock action), check if it's the scene's root and set edited scene if it is.

[Rindbee]

The choice to use signal was made in #68599, presumably because EditorNode::set_edited_scene() might automatically add new node to the tree, which would then fail when calling Node::replace_by().

godot/editor/editor_node.cpp

Lines 3726 to 3729 in 5f1184e

	if (p_scene) {
	if (p_scene->get_parent() != scene_root) {
	scene_root->add_child(p_scene, true);
	}

godot/scene/main/node.cpp

Line 2993 in 5f1184e

ERR_FAIL_COND(p_node->data.parent);

Node::replace_by() requires that the new node cannot have a parent node.

[KoBeWi]

So the solution is removing the node after setting it as scene 🙃

if (oldnode == edited_scene) {
	EditorNode::get_singleton()->set_edited_scene(newnode);
	newnode->get_parent()->remove_child(newnode);
}

It's best to avoid a core compatibility breakage due to editor issue, unless the new code has additional benefits (but it doesn't, no?).

[akien-mga]

Superseded by #92760.