Try to keep the terrain UI visible when editing generator and graph settings. Hacky stuff.

editor/graph/voxel_graph_node_inspector_wrapper.h

@@ -1,11 +1,9 @@
 #ifndef VOXEL_GRAPH_NODE_INSPECTOR_WRAPPER_H
 #define VOXEL_GRAPH_NODE_INSPECTOR_WRAPPER_H
 
-#include "../../generators/graph/program_graph.h"
+#include "../../generators/graph/voxel_generator_graph.h"
 #include <core/reference.h>
 
-class VoxelGeneratorGraph;
-
 // Nodes aren't resources so this translates them into a form the inspector can understand.
 // This makes it easier to support undo/redo and sub-resources.
 // WARNING: `AnimationPlayer` will allow to keyframe properties, but there really is no support for that.

editor/terrain/voxel_terrain_editor_plugin.cpp

@@ -1,6 +1,8 @@
 #include "voxel_terrain_editor_plugin.h"
+#include "../../generators/voxel_generator.h"
 #include "../../terrain/voxel_lod_terrain.h"
 #include "../../terrain/voxel_terrain.h"
+#include "../graph/voxel_graph_node_inspector_wrapper.h"
 
 VoxelTerrainEditorPlugin::VoxelTerrainEditorPlugin(EditorNode *p_node) {
 	_restart_stream_button = memnew(Button);
@@ -10,25 +12,78 @@ VoxelTerrainEditorPlugin::VoxelTerrainEditorPlugin(EditorNode *p_node) {
 	add_control_to_container(CONTAINER_SPATIAL_EDITOR_MENU, _restart_stream_button);
 }
 
-bool VoxelTerrainEditorPlugin::handles(Object *p_object) const {
+static Node *get_as_terrain(Object *p_object) {
 	VoxelTerrain *terrain = Object::cast_to<VoxelTerrain>(p_object);
 	if (terrain != nullptr) {
-		return true;
+		return terrain;
 	}
 	VoxelLodTerrain *terrain2 = Object::cast_to<VoxelLodTerrain>(p_object);
-	return terrain2 != nullptr;
+	if (terrain2 != nullptr) {
+		return terrain2;
+	}
+	return nullptr;
+}
+
+// Things the plugin doesn't directly work on, but still handles to keep things visible.
+// This is basically a hack because it's not easy to express that with EditorPlugin API.
+// The use case being, as long as we edit an object NESTED within a voxel terrain, we should keep things visible.
+static bool is_side_handled(Object *p_object) {
+	// Handle stream too so we can leave some controls visible while we edit a stream or generator
+	VoxelGenerator *generator = Object::cast_to<VoxelGenerator>(p_object);
+	if (generator != nullptr) {
+		return true;
+	}
+	// And have to account for this hack as well
+	VoxelGraphNodeInspectorWrapper *wrapper = Object::cast_to<VoxelGraphNodeInspectorWrapper>(p_object);
+	if (wrapper != nullptr) {
+		return true;
+	}
+	return false;
+}
+
+bool VoxelTerrainEditorPlugin::handles(Object *p_object) const {
+	if (get_as_terrain(p_object) != nullptr) {
+		return true;
+	}
+	if (_node != nullptr) {
+		return is_side_handled(p_object);
+	}
+	return false;
 }
 
 void VoxelTerrainEditorPlugin::edit(Object *p_object) {
-	_node = Object::cast_to<Node>(p_object);
+	Node *node = get_as_terrain(p_object);
+	if (node != nullptr) {
+		set_node(node);
+	} else {
+		if (!is_side_handled(p_object)) {
+			set_node(nullptr);
+		}
+	}
+}
+
+void VoxelTerrainEditorPlugin::set_node(Node *node) {
+	if (_node != nullptr) {
+		// Using this to know when the node becomes really invalid, because ObjectID is unreliable in Godot 3.x,
+		// and we may want to keep access to the node when we select some different kinds of objects.
+		// Also moving the node around in the tree triggers exit/enter so have to listen for both.
+		_node->disconnect("tree_entered", this, "_on_terrain_tree_entered");
+		_node->disconnect("tree_exited", this, "_on_terrain_tree_exited");
+	}
+
+	_node = node;
+
+	if (_node != nullptr) {
+		_node->connect("tree_entered", this, "_on_terrain_tree_entered", varray(_node));
+		_node->connect("tree_exited", this, "_on_terrain_tree_exited", varray(_node));
+	}
 }
 
 void VoxelTerrainEditorPlugin::make_visible(bool visible) {
 	_restart_stream_button->set_visible(visible);
-
-	if (!visible) {
-		_node = nullptr;
-	}
+	// Can't use `make_visible(false)` to reset our reference to the node,
+	// because of https://github.com/godotengine/godot/issues/40166
+	// So we'll need to check if _node is null all over the place
 }
 
 void VoxelTerrainEditorPlugin::_on_restart_stream_button_pressed() {
@@ -43,7 +98,18 @@ void VoxelTerrainEditorPlugin::_on_restart_stream_button_pressed() {
 	terrain2->restart_stream();
 }
 
+void VoxelTerrainEditorPlugin::_on_terrain_tree_entered(Node *node) {
+	// If the node exited the tree because it was deleted, signals we connected should automatically disconnect.
+	_node = node;
+}
+
+void VoxelTerrainEditorPlugin::_on_terrain_tree_exited(Node *node) {
+	_node = nullptr;
+}
+
 void VoxelTerrainEditorPlugin::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("_on_restart_stream_button_pressed"),
 			&VoxelTerrainEditorPlugin::_on_restart_stream_button_pressed);
+	ClassDB::bind_method(D_METHOD("_on_terrain_tree_entered"), &VoxelTerrainEditorPlugin::_on_terrain_tree_entered);
+	ClassDB::bind_method(D_METHOD("_on_terrain_tree_exited"), &VoxelTerrainEditorPlugin::_on_terrain_tree_exited);
 }

editor/terrain/voxel_terrain_editor_plugin.h

@@ -15,7 +15,11 @@ public:
 	void make_visible(bool visible) override;
 
 private:
+	void set_node(Node *node);
+
 	void _on_restart_stream_button_pressed();
+	void _on_terrain_tree_entered(Node *node);
+	void _on_terrain_tree_exited(Node *node);
 
 	static void _bind_methods();