Improve the 3D editor selection box appearance

- Draw two boxes slightly offset from each other to give the illustion
  of a thicker outline.
- Decrease the offset compared to the 3D node's AABB to give a more
  accurate representation of its size.
- Make the box fully visible instead of only displaying the corners.
- Draw a x-ray version of the box that's more translucent, but visible
  through walls. This helps make the box more visible while still
  having a sense of depth.
- Use an orange color similar to the 2D editor.

editor/plugins/node_3d_editor_plugin.cpp

@@ -2426,6 +2426,7 @@ void Node3DEditorViewport::_notification(int p_what) {
 			t.basis = t.basis * aabb_s;
 
 			RenderingServer::get_singleton()->instance_set_transform(se->sbox_instance, t);
+			RenderingServer::get_singleton()->instance_set_transform(se->sbox_instance_xray, t);
 		}
 
 		if (changed || (spatial_editor->is_gizmo_visible() && !exist)) {
@@ -4417,6 +4418,9 @@ Node3DEditorSelectedItem::~Node3DEditorSelectedItem() {
 	if (sbox_instance.is_valid()) {
 		RenderingServer::get_singleton()->free(sbox_instance);
 	}
+	if (sbox_instance_xray.is_valid()) {
+		RenderingServer::get_singleton()->free(sbox_instance_xray);
+	}
 }
 
 void Node3DEditor::select_gizmo_highlight_axis(int p_axis) {
@@ -4500,42 +4504,73 @@ Object *Node3DEditor::_get_editor_data(Object *p_what) {
 	Node3DEditorSelectedItem *si = memnew(Node3DEditorSelectedItem);
 
 	si->sp = sp;
-	si->sbox_instance = RenderingServer::get_singleton()->instance_create2(selection_box->get_rid(), sp->get_world_3d()->get_scenario());
-	RS::get_singleton()->instance_geometry_set_cast_shadows_setting(si->sbox_instance, RS::SHADOW_CASTING_SETTING_OFF);
+	si->sbox_instance = RenderingServer::get_singleton()->instance_create2(
+			selection_box->get_rid(),
+			sp->get_world_3d()->get_scenario());
+	RS::get_singleton()->instance_geometry_set_cast_shadows_setting(
+			si->sbox_instance,
+			RS::SHADOW_CASTING_SETTING_OFF);
+	si->sbox_instance_xray = RenderingServer::get_singleton()->instance_create2(
+			selection_box_xray->get_rid(),
+			sp->get_world_3d()->get_scenario());
+	RS::get_singleton()->instance_geometry_set_cast_shadows_setting(
+			si->sbox_instance_xray,
+			RS::SHADOW_CASTING_SETTING_OFF);
 
 	return si;
 }
 
-void Node3DEditor::_generate_selection_box() {
+void Node3DEditor::_generate_selection_boxes() {
+	// Use two AABBs to create the illusion of a slightly thicker line.
 	AABB aabb(Vector3(), Vector3(1, 1, 1));
-	aabb.grow_by(aabb.get_longest_axis_size() / 20.0);
-
+	AABB aabb_offset(Vector3(), Vector3(1, 1, 1));
+	// Grow the bounding boxes slightly to avoid Z-fighting with the mesh's edges.
+	aabb.grow_by(0.005);
+	aabb_offset.grow_by(0.01);
+
+	// Create a x-ray (visible through solid surfaces) and standard version of the selection box.
+	// Both will be drawn at the same position, but with different opacity.
+	// This lets the user see where the selection is while still having a sense of depth.
 	Ref<SurfaceTool> st = memnew(SurfaceTool);
+	Ref<SurfaceTool> st_xray = memnew(SurfaceTool);
 
 	st->begin(Mesh::PRIMITIVE_LINES);
+	st_xray->begin(Mesh::PRIMITIVE_LINES);
 	for (int i = 0; i < 12; i++) {
 		Vector3 a, b;
 		aabb.get_edge(i, a, b);
 
-		st->add_color(Color(1.0, 1.0, 0.8, 0.8));
 		st->add_vertex(a);
-		st->add_color(Color(1.0, 1.0, 0.8, 0.4));
-		st->add_vertex(a.lerp(b, 0.2));
+		st->add_vertex(b);
+		st_xray->add_vertex(a);
+		st_xray->add_vertex(b);
+	}
 
-		st->add_color(Color(1.0, 1.0, 0.8, 0.4));
-		st->add_vertex(a.lerp(b, 0.8));
-		st->add_color(Color(1.0, 1.0, 0.8, 0.8));
+	for (int i = 0; i < 12; i++) {
+		Vector3 a, b;
+		aabb_offset.get_edge(i, a, b);
+
+		st->add_vertex(a);
 		st->add_vertex(b);
+		st_xray->add_vertex(a);
+		st_xray->add_vertex(b);
 	}
 
 	Ref<StandardMaterial3D> mat = memnew(StandardMaterial3D);
 	mat->set_shading_mode(StandardMaterial3D::SHADING_MODE_UNSHADED);
-	mat->set_albedo(Color(1, 1, 1));
+	// Use a similar color to the 2D editor selection.
+	mat->set_albedo(Color(1, 0.5, 0));
 	mat->set_transparency(StandardMaterial3D::TRANSPARENCY_ALPHA);
-	mat->set_flag(StandardMaterial3D::FLAG_ALBEDO_FROM_VERTEX_COLOR, true);
-	mat->set_flag(StandardMaterial3D::FLAG_SRGB_VERTEX_COLOR, true);
 	st->set_material(mat);
 	selection_box = st->commit();
+
+	Ref<StandardMaterial3D> mat_xray = memnew(StandardMaterial3D);
+	mat_xray->set_shading_mode(StandardMaterial3D::SHADING_MODE_UNSHADED);
+	mat_xray->set_flag(StandardMaterial3D::FLAG_DISABLE_DEPTH_TEST, true);
+	mat_xray->set_albedo(Color(1, 0.5, 0, 0.15));
+	mat_xray->set_transparency(StandardMaterial3D::TRANSPARENCY_ALPHA);
+	st_xray->set_material(mat_xray);
+	selection_box_xray = st_xray->commit();
 }
 
 Dictionary Node3DEditor::get_state() const {
@@ -5514,7 +5549,7 @@ void Node3DEditor::_init_indicators() {
 		}
 	}
 
-	_generate_selection_box();
+	_generate_selection_boxes();
 }
 
 void Node3DEditor::_update_gizmos_menu() {

editor/plugins/node_3d_editor_plugin.h

@@ -498,6 +498,7 @@ public:
 	bool last_xform_dirty;
 	Node3D *sp;
 	RID sbox_instance;
+	RID sbox_instance_xray;
 
 	Node3DEditorSelectedItem() {
 		sp = nullptr;
@@ -613,6 +614,7 @@ private:
 	float snap_rotate_value;
 	float snap_scale_value;
 
+	Ref<ArrayMesh> selection_box_xray;
 	Ref<ArrayMesh> selection_box;
 	RID indicators;
 	RID indicators_instance;
@@ -701,7 +703,7 @@ private:
 
 	HBoxContainer *hbc_menu;
 
-	void _generate_selection_box();
+	void _generate_selection_boxes();
 	UndoRedo *undo_redo;
 
 	int camera_override_viewport_id;