Perform update_mouse_cursor_state at most once per frame.

doc/classes/Viewport.xml

@@ -217,7 +217,7 @@
 		<method name="update_mouse_cursor_state">
 			<return type="void" />
 			<description>
-				Force instantly updating the display based on the current mouse cursor position. This includes updating the mouse cursor shape and sending necessary [signal Control.mouse_entered], [signal CollisionObject2D.mouse_entered], [signal CollisionObject3D.mouse_entered] and [signal Window.mouse_entered] signals and their respective [code]mouse_exited[/code] counterparts.
+				Force updating the display based on the current mouse cursor position at the end of the current frame. This includes updating the mouse cursor shape and sending necessary [signal Control.mouse_entered], [signal CollisionObject2D.mouse_entered], [signal CollisionObject3D.mouse_entered] and [signal Window.mouse_entered] signals and their respective [code]mouse_exited[/code] counterparts.
 			</description>
 		</method>
 		<method name="warp_mouse">

scene/main/window.cpp

@@ -721,11 +721,16 @@ void Window::_event_callback(DisplayServer::WindowEvent p_event) {
 
 void Window::update_mouse_cursor_state() {
 	ERR_MAIN_THREAD_GUARD;
-	// Update states based on mouse cursor position.
+	// Flag the mouse cursor state to be updated at the end of the frame based on mouse cursor position.
 	// This includes updated mouse_enter or mouse_exit signals or the current mouse cursor shape.
 	// These details are set in Viewport::_gui_input_event. To instantly
 	// see the changes in the viewport, we need to trigger a mouse motion event.
 	// This function should be called whenever scene tree changes affect the mouse cursor.
+
+	pending_mouse_cursor_state_update = true;
+}
+
+void Window::_update_mouse_cursor_state() {
 	Ref<InputEventMouseMotion> mm;
 	Vector2 pos = get_mouse_position();
 	Transform2D xform = get_global_canvas_transform().affine_inverse();
@@ -1274,6 +1279,14 @@ void Window::_notification(int p_what) {
 
 			RS::get_singleton()->viewport_set_active(get_viewport_rid(), false);
 		} break;
+
+		case NOTIFICATION_PROCESS: {
+			// TODO: remove this section and disable processing when update_mouse_cursor_state no longer creates InputEvents.
+			if (pending_mouse_cursor_state_update) {
+				_update_mouse_cursor_state();
+				pending_mouse_cursor_state_update = false;
+			}
+		} break;
 	}
 }
 
@@ -2737,6 +2750,7 @@ Window::Window() {
 		max_size_used = max_size; // Update max_size_used.
 	}
 
+	set_process(true);
 	theme_owner = memnew(ThemeOwner);
 	RS::get_singleton()->viewport_set_update_mode(get_viewport_rid(), RS::VIEWPORT_UPDATE_DISABLED);
 }

scene/main/window.h

@@ -196,6 +196,8 @@ class Window : public Viewport {
 	virtual bool _can_consume_input_events() const override;
 
 	Ref<Shortcut> debugger_stop_shortcut;
+	bool pending_mouse_cursor_state_update = false;
+	void _update_mouse_cursor_state();
 
 protected:
 	virtual Rect2i _popup_adjust_rect() const { return Rect2i(); }

tests/scene/test_color_picker.h

@@ -0,0 +1,61 @@
+/**************************************************************************/
+/*  test_color_picker.h                                                   */
+/**************************************************************************/
+/*                         This file is part of:                          */
+/*                             GODOT ENGINE                               */
+/*                        https://godotengine.org                         */
+/**************************************************************************/
+/* Copyright (c) 2014-present Godot Engine contributors (see AUTHORS.md). */
+/* Copyright (c) 2007-2014 Juan Linietsky, Ariel Manzur.                  */
+/*                                                                        */
+/* Permission is hereby granted, free of charge, to any person obtaining  */
+/* a copy of this software and associated documentation files (the        */
+/* "Software"), to deal in the Software without restriction, including    */
+/* without limitation the rights to use, copy, modify, merge, publish,    */
+/* distribute, sublicense, and/or sell copies of the Software, and to     */
+/* permit persons to whom the Software is furnished to do so, subject to  */
+/* the following conditions:                                              */
+/*                                                                        */
+/* The above copyright notice and this permission notice shall be         */
+/* included in all copies or substantial portions of the Software.        */
+/*                                                                        */
+/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,        */
+/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF     */
+/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. */
+/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY   */
+/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,   */
+/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE      */
+/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                 */
+/**************************************************************************/
+
+#ifndef TEST_COLOR_PICKER_H
+#define TEST_COLOR_PICKER_H
+
+#include "scene/gui/color_picker.h"
+
+#include "tests/test_macros.h"
+
+namespace TestColorPicker {
+
+TEST_CASE("[SceneTree][ColorPicker]") {
+	ColorPicker *cp = memnew(ColorPicker);
+	Window *root = SceneTree::get_singleton()->get_root();
+	root->add_child(cp);
+
+	SUBCASE("[COLOR_PICKER] Mouse movement after Slider release") {
+		Point2i pos_left = Point2i(50, 340); // On the left side of the red slider.
+		Point2i pos_right = Point2i(200, 340); // On the right side of the red slider.
+		SEND_GUI_MOUSE_MOTION_EVENT(pos_left, MouseButtonMask::NONE, Key::NONE);
+		SEND_GUI_MOUSE_BUTTON_EVENT(pos_left, MouseButton::LEFT, MouseButtonMask::LEFT, Key::NONE);
+		CHECK(cp->get_pick_color().r < 0.5);
+		SEND_GUI_MOUSE_MOTION_EVENT(pos_right, MouseButtonMask::LEFT, Key::NONE);
+		CHECK(cp->get_pick_color().r > 0.5);
+		SEND_GUI_MOUSE_BUTTON_RELEASED_EVENT(pos_right, MouseButton::LEFT, MouseButtonMask::NONE, Key::NONE);
+		SEND_GUI_MOUSE_MOTION_EVENT(pos_left, MouseButtonMask::NONE, Key::NONE);
+		CHECK(cp->get_pick_color().r > 0.5); // Issue GH-77773.
+	}
+}
+
+} // namespace TestColorPicker
+
+#endif // TEST_COLOR_PICKER_H

tests/test_main.cpp

@@ -91,6 +91,7 @@
 #include "tests/scene/test_audio_stream_wav.h"
 #include "tests/scene/test_bit_map.h"
 #include "tests/scene/test_code_edit.h"
+#include "tests/scene/test_color_picker.h"
 #include "tests/scene/test_curve.h"
 #include "tests/scene/test_curve_2d.h"
 #include "tests/scene/test_curve_3d.h"