Add support for multiple top-level windows on Windows

ci/licenses_golden/excluded_files

@@ -405,6 +405,7 @@
 ../../../flutter/shell/platform/windows/direct_manipulation_unittests.cc
 ../../../flutter/shell/platform/windows/dpi_utils_unittests.cc
 ../../../flutter/shell/platform/windows/fixtures
+../../../flutter/shell/platform/windows/flutter_host_window_controller_unittests.cc
 ../../../flutter/shell/platform/windows/flutter_project_bundle_unittests.cc
 ../../../flutter/shell/platform/windows/flutter_window_unittests.cc
 ../../../flutter/shell/platform/windows/flutter_windows_engine_unittests.cc
@@ -425,6 +426,7 @@
 ../../../flutter/shell/platform/windows/text_input_plugin_unittest.cc
 ../../../flutter/shell/platform/windows/window_proc_delegate_manager_unittests.cc
 ../../../flutter/shell/platform/windows/window_unittests.cc
+../../../flutter/shell/platform/windows/windowing_handler_unittests.cc
 ../../../flutter/shell/platform/windows/windows_lifecycle_manager_unittests.cc
 ../../../flutter/shell/profiling/sampling_profiler_unittest.cc
 ../../../flutter/shell/testing

ci/licenses_golden/licenses_flutter

@@ -47462,6 +47462,7 @@ FILE: ../../../flutter/shell/platform/common/text_editing_delta.h
 FILE: ../../../flutter/shell/platform/common/text_input_model.cc
 FILE: ../../../flutter/shell/platform/common/text_input_model.h
 FILE: ../../../flutter/shell/platform/common/text_range.h
+FILE: ../../../flutter/shell/platform/common/windowing.h
 FILE: ../../../flutter/shell/platform/darwin/common/availability_version_check.cc
 FILE: ../../../flutter/shell/platform/darwin/common/availability_version_check.h
 FILE: ../../../flutter/shell/platform/darwin/common/buffer_conversions.h
@@ -48208,6 +48209,10 @@ FILE: ../../../flutter/shell/platform/windows/flutter_platform_node_delegate_win
 FILE: ../../../flutter/shell/platform/windows/flutter_platform_node_delegate_windows.h
 FILE: ../../../flutter/shell/platform/windows/flutter_project_bundle.cc
 FILE: ../../../flutter/shell/platform/windows/flutter_project_bundle.h
+FILE: ../../../flutter/shell/platform/windows/flutter_host_window.cc
+FILE: ../../../flutter/shell/platform/windows/flutter_host_window.h
+FILE: ../../../flutter/shell/platform/windows/flutter_host_window_controller.cc
+FILE: ../../../flutter/shell/platform/windows/flutter_host_window_controller.h
 FILE: ../../../flutter/shell/platform/windows/flutter_window.cc
 FILE: ../../../flutter/shell/platform/windows/flutter_window.h
 FILE: ../../../flutter/shell/platform/windows/flutter_windows.cc
@@ -48257,6 +48262,8 @@ FILE: ../../../flutter/shell/platform/windows/window_binding_handler_delegate.h
 FILE: ../../../flutter/shell/platform/windows/window_proc_delegate_manager.cc
 FILE: ../../../flutter/shell/platform/windows/window_proc_delegate_manager.h
 FILE: ../../../flutter/shell/platform/windows/window_state.h
+FILE: ../../../flutter/shell/platform/windows/windowing_handler.cc
+FILE: ../../../flutter/shell/platform/windows/windowing_handler.h
 FILE: ../../../flutter/shell/platform/windows/windows_lifecycle_manager.cc
 FILE: ../../../flutter/shell/platform/windows/windows_lifecycle_manager.h
 FILE: ../../../flutter/shell/platform/windows/windows_proc_table.cc

common/settings.h

@@ -367,6 +367,10 @@ struct Settings {
   // If true, the UI thread is the platform thread on supported
   // platforms.
   bool merged_platform_ui_thread = true;
+
+  // Enable support for multiple windows. Ignored if not supported on the
+  // platform.
+  bool enable_multi_window = false;
 };
 
 }  // namespace flutter

shell/common/switches.cc

@@ -532,6 +532,17 @@ Settings SettingsFromCommandLine(const fml::CommandLine& command_line) {
   settings.merged_platform_ui_thread = !command_line.HasOption(
       FlagForSwitch(Switch::DisableMergedPlatformUIThread));
 
+#if FML_OS_WIN
+  // Process the EnableMultiWindow switch on Windows.
+  {
+    std::string enable_multi_window_value;
+    if (command_line.GetOptionValue(FlagForSwitch(Switch::EnableMultiWindow),
+                                    &enable_multi_window_value)) {
+      settings.enable_multi_window = "true" == enable_multi_window_value;
+    }
+  }
+#endif  // FML_OS_WIN
+
   return settings;
 }
 

shell/common/switches.h

@@ -300,6 +300,10 @@ DEF_SWITCH(DisableMergedPlatformUIThread,
 DEF_SWITCH(DisableAndroidSurfaceControl,
            "disable-surface-control",
            "Disable the SurfaceControl backed swapchain even when supported.")
+DEF_SWITCH(EnableMultiWindow,
+           "enable-multi-window",
+           "Enable support for multiple windows. Ignored if not supported on "
+           "the platform.")
 DEF_SWITCHES_END
 
 void PrintUsage(const std::string& executable_name);

shell/platform/common/BUILD.gn

@@ -142,6 +142,7 @@ source_set("common_cpp_core") {
   public = [
     "geometry.h",
     "path_utils.h",
+    "windowing.h",
   ]
 
   sources = [ "path_utils.cc" ]

shell/platform/common/windowing.h

@@ -0,0 +1,87 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef FLUTTER_SHELL_PLATFORM_COMMON_WINDOWING_H_
+#define FLUTTER_SHELL_PLATFORM_COMMON_WINDOWING_H_
+
+#include <optional>
+
+namespace flutter {
+
+// A unique identifier for a view.
+using FlutterViewId = int64_t;
+
+// A point in 2D space for window positioning using integer coordinates.
+struct WindowPoint {
+  int x = 0;
+  int y = 0;
+
+  friend WindowPoint operator+(WindowPoint const& lhs, WindowPoint const& rhs) {
+    return {lhs.x + rhs.x, lhs.y + rhs.y};
+  }
+
+  friend WindowPoint operator-(WindowPoint const& lhs, WindowPoint const& rhs) {
+    return {lhs.x - rhs.x, lhs.y - rhs.y};
+  }
+
+  friend bool operator==(WindowPoint const& lhs, WindowPoint const& rhs) {
+    return lhs.x == rhs.x && lhs.y == rhs.y;
+  }
+};
+
+// A 2D size using integer dimensions.
+struct WindowSize {
+  int width = 0;
+  int height = 0;
+
+  explicit operator WindowPoint() const { return {width, height}; }
+
+  friend bool operator==(WindowSize const& lhs, WindowSize const& rhs) {
+    return lhs.width == rhs.width && lhs.height == rhs.height;
+  }
+};
+
+// A rectangular area defined by a top-left point and size.
+struct WindowRectangle {
+  WindowPoint top_left;
+  WindowSize size;
+
+  // Checks if this rectangle fully contains |rect|.
+  // Note: An empty rectangle can still contain other empty rectangles,
+  // which are treated as points or lines of thickness zero
+  bool contains(WindowRectangle const& rect) const {
+    return rect.top_left.x >= top_left.x &&
+           rect.top_left.x + rect.size.width <= top_left.x + size.width &&
+           rect.top_left.y >= top_left.y &&
+           rect.top_left.y + rect.size.height <= top_left.y + size.height;
+  }
+
+  friend bool operator==(WindowRectangle const& lhs,
+                         WindowRectangle const& rhs) {
+    return lhs.top_left == rhs.top_left && lhs.size == rhs.size;
+  }
+};
+
+// Types of windows.
+enum class WindowArchetype {
+  // Regular top-level window.
+  regular,
+};
+
+// Window metadata returned as the result of creating a Flutter window.
+struct WindowMetadata {
+  // The ID of the view used for this window, which is unique to each window.
+  FlutterViewId view_id = 0;
+  // The type of the window.
+  WindowArchetype archetype = WindowArchetype::regular;
+  // Size of the created window, in logical coordinates.
+  WindowSize size;
+  // The ID of the view used by the parent window. If not set, the window is
+  // assumed a top-level window.
+  std::optional<FlutterViewId> parent_id;
+};
+
+}  // namespace flutter
+
+#endif  // FLUTTER_SHELL_PLATFORM_COMMON_WINDOWING_H_

shell/platform/windows/BUILD.gn

@@ -73,6 +73,10 @@ source_set("flutter_windows_source") {
     "external_texture_d3d.h",
     "external_texture_pixelbuffer.cc",
     "external_texture_pixelbuffer.h",
+    "flutter_host_window.cc",
+    "flutter_host_window.h",
+    "flutter_host_window_controller.cc",
+    "flutter_host_window_controller.h",
     "flutter_key_map.g.cc",
     "flutter_platform_node_delegate_windows.cc",
     "flutter_platform_node_delegate_windows.h",
@@ -125,6 +129,8 @@ source_set("flutter_windows_source") {
     "window_proc_delegate_manager.cc",
     "window_proc_delegate_manager.h",
     "window_state.h",
+    "windowing_handler.cc",
+    "windowing_handler.h",
     "windows_lifecycle_manager.cc",
     "windows_lifecycle_manager.h",
     "windows_proc_table.cc",
@@ -202,6 +208,7 @@ executable("flutter_windows_unittests") {
     "cursor_handler_unittests.cc",
     "direct_manipulation_unittests.cc",
     "dpi_utils_unittests.cc",
+    "flutter_host_window_controller_unittests.cc",
     "flutter_project_bundle_unittests.cc",
     "flutter_window_unittests.cc",
     "flutter_windows_engine_unittests.cc",
@@ -249,6 +256,7 @@ executable("flutter_windows_unittests") {
     "text_input_plugin_unittest.cc",
     "window_proc_delegate_manager_unittests.cc",
     "window_unittests.cc",
+    "windowing_handler_unittests.cc",
     "windows_lifecycle_manager_unittests.cc",
   ]