Calculate the mouse focus inverse transform only when needed

[madmiraal]

Currently, gui.focus_inv_xform is used to determine the location of an event within a Control. Viewport::_gui_find_control() has the side-effect of updating gui.focus_inv_xform. This would be fine if _gui_find_control() was just used to assign gui.mouse_focus, but it's not. The result is, when a mouse button release event happens, sometimes gui.focus_inv_xform does not provide the location of the event inside the Control where the mouse button press event happened.

Instead of calculating the mouse_focus's Control's inverse transform every time _gui_find_control() is called and storing the result for later use, this PR calculates the required inverse transform only when it is needed. This will not only prevent the wrong inverse transform sometimes being used, but should result in better performance too.

Fixes #41585.

[Sauermann]

I have resolved the merge-conflict of this PR and it seems to work in current master:

diff --git a/scene/main/viewport.cpp b/scene/main/viewport.cpp
index 43a2c9473e..9374cdebe2 100644
--- a/scene/main/viewport.cpp
+++ b/scene/main/viewport.cpp
@@ -1395,7 +1395,7 @@ Control *Viewport::gui_find_control(const Point2 &p_global) {
 			xform = sw->get_canvas_transform();
 		}
 
-		Control *ret = _gui_find_control_at_pos(sw, p_global, xform, gui.focus_inv_xform);
+		Control *ret = _gui_find_control_at_pos(sw, p_global, xform);
 		if (ret) {
 			return ret;
 		}
@@ -1404,7 +1404,7 @@ Control *Viewport::gui_find_control(const Point2 &p_global) {
 	return nullptr;
 }
 
-Control *Viewport::_gui_find_control_at_pos(CanvasItem *p_node, const Point2 &p_global, const Transform2D &p_xform, Transform2D &r_inv_xform) {
+Control *Viewport::_gui_find_control_at_pos(CanvasItem *p_node, const Point2 &p_global, const Transform2D &p_xform) {
 	if (!p_node->is_visible()) {
 		return nullptr; // Canvas item hidden, discard.
 	}
@@ -1424,7 +1424,7 @@ Control *Viewport::_gui_find_control_at_pos(CanvasItem *p_node, const Point2 &p_
 				continue;
 			}
 
-			Control *ret = _gui_find_control_at_pos(ci, p_global, matrix, r_inv_xform);
+			Control *ret = _gui_find_control_at_pos(ci, p_global, matrix);
 			if (ret) {
 				return ret;
 			}
@@ -1442,7 +1442,6 @@ Control *Viewport::_gui_find_control_at_pos(CanvasItem *p_node, const Point2 &p_
 
 	Control *drag_preview = _gui_get_drag_preview();
 	if (!drag_preview || (c != drag_preview && !drag_preview->is_ancestor_of(c))) {
-		r_inv_xform = matrix;
 		return c;
 	}
 
@@ -1489,12 +1488,11 @@ void Viewport::_gui_input_event(Ref<InputEvent> p_event) {
 
 		Point2 mpos = mb->get_position();
 		if (mb->is_pressed()) {
-			Size2 pos = mpos;
 			if (gui.mouse_focus_mask != MouseButton::NONE) {
 				// Do not steal mouse focus and stuff while a focus mask exists.
 				gui.mouse_focus_mask |= mouse_button_to_mask(mb->get_button_index());
 			} else {
-				gui.mouse_focus = gui_find_control(pos);
+				gui.mouse_focus = gui_find_control(mpos);
 				gui.last_mouse_focus = gui.mouse_focus;
 
 				if (!gui.mouse_focus) {
@@ -1512,10 +1510,7 @@ void Viewport::_gui_input_event(Ref<InputEvent> p_event) {
 
 			mb = mb->xformed_by(Transform2D()); // Make a copy of the event.
 
-			mb->set_global_position(pos);
-
-			pos = gui.focus_inv_xform.xform(pos);
-
+			Point2 pos = gui.mouse_focus->get_global_transform_with_canvas().affine_inverse().xform(mpos);
 			mb->set_position(pos);
 
 #ifdef DEBUG_ENABLED
@@ -1579,11 +1574,8 @@ void Viewport::_gui_input_event(Ref<InputEvent> p_event) {
 				return;
 			}
 
-			Size2 pos = mpos;
-
 			mb = mb->xformed_by(Transform2D()); // Make a copy.
-			mb->set_global_position(pos);
-			pos = gui.focus_inv_xform.xform(pos);
+			Point2 pos = gui.mouse_focus->get_global_transform_with_canvas().affine_inverse().xform(mpos);
 			mb->set_position(pos);
 
 			Control *mouse_focus = gui.mouse_focus;
@@ -1874,18 +1866,14 @@ void Viewport::_gui_input_event(Ref<InputEvent> p_event) {
 
 	Ref<InputEventScreenTouch> touch_event = p_event;
 	if (touch_event.is_valid()) {
-		Size2 pos = touch_event->get_position();
+		Point2 pos = touch_event->get_position();
 		if (touch_event->is_pressed()) {
 			Control *over = gui_find_control(pos);
 			if (over) {
 				bool stopped = false;
 				if (over->can_process()) {
 					touch_event = touch_event->xformed_by(Transform2D()); // Make a copy.
-					if (over == gui.mouse_focus) {
-						pos = gui.focus_inv_xform.xform(pos);
-					} else {
-						pos = over->get_global_transform_with_canvas().affine_inverse().xform(pos);
-					}
+					pos = over->get_global_transform_with_canvas().affine_inverse().xform(pos);
 					touch_event->set_position(pos);
 					stopped = _gui_call_input(over, touch_event);
 				}
@@ -1898,7 +1886,8 @@ void Viewport::_gui_input_event(Ref<InputEvent> p_event) {
 			bool stopped = false;
 			if (gui.last_mouse_focus->can_process()) {
 				touch_event = touch_event->xformed_by(Transform2D()); // Make a copy.
-				touch_event->set_position(gui.focus_inv_xform.xform(pos));
+				pos = gui.last_mouse_focus->get_global_transform_with_canvas().affine_inverse().xform(pos);
+				touch_event->set_position(pos);
 
 				stopped = _gui_call_input(gui.last_mouse_focus, touch_event);
 			}
@@ -1922,11 +1911,7 @@ void Viewport::_gui_input_event(Ref<InputEvent> p_event) {
 			bool stopped = false;
 			if (over->can_process()) {
 				gesture_event = gesture_event->xformed_by(Transform2D()); // Make a copy.
-				if (over == gui.mouse_focus) {
-					pos = gui.focus_inv_xform.xform(pos);
-				} else {
-					pos = over->get_global_transform_with_canvas().affine_inverse().xform(pos);
-				}
+				pos = over->get_global_transform_with_canvas().affine_inverse().xform(pos);
 				gesture_event->set_position(pos);
 				stopped = _gui_call_input(over, gesture_event);
 			}
@@ -2371,7 +2356,6 @@ void Viewport::_post_gui_grab_click_focus() {
 		}
 
 		gui.mouse_focus = focus_grabber;
-		gui.focus_inv_xform = gui.mouse_focus->get_global_transform_with_canvas().affine_inverse();
 		click = gui.mouse_focus->get_global_transform_with_canvas().affine_inverse().xform(gui.last_mouse_pos);
 
 		for (int i = 0; i < 3; i++) {
diff --git a/scene/main/viewport.h b/scene/main/viewport.h
index dc69ec24d8..7cadfc7541 100644
--- a/scene/main/viewport.h
+++ b/scene/main/viewport.h
@@ -373,7 +373,6 @@ private:
 		ObjectID drag_preview_id;
 		Ref<SceneTreeTimer> tooltip_timer;
 		double tooltip_delay = 0.0;
-		Transform2D focus_inv_xform;
 		bool roots_order_dirty = false;
 		List<Control *> roots;
 		int canvas_sort_index = 0; //for sorting items with canvas as root
@@ -402,7 +401,7 @@ private:
 	void _gui_call_notification(Control *p_control, int p_what);
 
 	void _gui_sort_roots();
-	Control *_gui_find_control_at_pos(CanvasItem *p_node, const Point2 &p_global, const Transform2D &p_xform, Transform2D &r_inv_xform);
+	Control *_gui_find_control_at_pos(CanvasItem *p_node, const Point2 &p_global, const Transform2D &p_xform);
 
 	void _gui_input_event(Ref<InputEvent> p_event);
 	void _perform_drop(Control *p_control = nullptr, Point2 p_pos = Point2());

[akien-mga]

Superseded by #69598.