Fix ImGui sometimes not respecting the viewport resolution in-Editor

ImguiAtomSystemComponent isn't guaranteed to initialize before ImGuiManager, which caused some issues.
Additionally, because we allow the user to configure ImGui's render resolution, I've refactored the window size override into a new OverrideRenderWindowSize API in ImGuiManager to decouple it from the target render resolution.

Gems/AtomLyIntegration/ImguiAtom/Code/Source/ImguiAtomSystemComponent.cpp

@@ -59,14 +59,8 @@ namespace AZ
             const AZ::Name contextName = atomViewportRequests->GetDefaultViewportContextName();
             AZ::RPI::ViewportContextNotificationBus::Handler::BusConnect(contextName);
 
-#if defined(IMGUI_ENABLED)
-            ImGui::ImGuiManagerListenerBus::Broadcast(&ImGui::IImGuiManagerListener::SetResolutionMode, ImGui::ImGuiResolutionMode::LockToResolution);
-            auto defaultViewportContext = atomViewportRequests->GetDefaultViewportContext();
-            if (defaultViewportContext)
-            {
-                OnViewportSizeChanged(defaultViewportContext->GetViewportSize());
-            }
-#endif
+            m_initialized = false;
+            InitializeViewportSizeIfNeeded();
         }
 
         void ImguiAtomSystemComponent::Deactivate()
@@ -75,6 +69,23 @@ namespace AZ
             AZ::RPI::ViewportContextNotificationBus::Handler::BusDisconnect();
         }
 
+        void ImguiAtomSystemComponent::InitializeViewportSizeIfNeeded()
+        {
+#if defined(IMGUI_ENABLED)
+            if (m_initialized)
+            {
+                return;
+            }
+            auto atomViewportRequests = AZ::Interface<AZ::RPI::ViewportContextRequestsInterface>::Get();
+            auto defaultViewportContext = atomViewportRequests->GetDefaultViewportContext();
+            if (defaultViewportContext)
+            {
+                // If this succeeds, m_initialized will be set to true.
+                OnViewportSizeChanged(defaultViewportContext->GetViewportSize());
+            }
+#endif
+        }
+
         void ImguiAtomSystemComponent::RenderImGuiBuffers(const ImDrawData& drawData)
         {
             Render::ImGuiSystemRequestBus::Broadcast(&Render::ImGuiSystemRequests::RenderImGuiBuffersToCurrentViewport, drawData);
@@ -83,6 +94,7 @@ namespace AZ
         void ImguiAtomSystemComponent::OnRenderTick()
         {
 #if defined(IMGUI_ENABLED)
+            InitializeViewportSizeIfNeeded();
             ImGui::ImGuiManagerListenerBus::Broadcast(&ImGui::IImGuiManagerListener::Render);
 #endif
         }
@@ -90,7 +102,17 @@ namespace AZ
         void ImguiAtomSystemComponent::OnViewportSizeChanged(AzFramework::WindowSize size)
         {
 #if defined(IMGUI_ENABLED)
-            ImGui::ImGuiManagerListenerBus::Broadcast(&ImGui::IImGuiManagerListener::SetImGuiRenderResolution, ImVec2{aznumeric_cast<float>(size.m_width), aznumeric_cast<float>(size.m_height)});
+            ImGui::ImGuiManagerListenerBus::Broadcast([this, size](ImGui::ImGuiManagerListenerBus::Events* imgui)
+            {
+                imgui->OverrideRenderWindowSize(size.m_width, size.m_height);
+                // ImGuiManagerListenerBus may not have been connected when this system component is activated
+                // as ImGuiManager is not part of a system component we can  require and instead just listens for ESYSTEM_EVENT_GAME_POST_INIT.
+                // Let our ImguiAtomSystemComponent know once we successfully connect and update the viewport size.
+                if (!m_initialized)
+                {
+                    m_initialized = true;
+                }
+            });
 #endif
         }
     }

Gems/AtomLyIntegration/ImguiAtom/Code/Source/ImguiAtomSystemComponent.h

@@ -48,6 +48,7 @@ namespace AZ
             void Deactivate() override;
 
         private:
+            void InitializeViewportSizeIfNeeded();
 
             // OtherActiveImGuiRequestBus overrides ...
             void RenderImGuiBuffers(const ImDrawData& drawData) override;
@@ -57,6 +58,7 @@ namespace AZ
             void OnViewportSizeChanged(AzFramework::WindowSize size) override;
 
             DebugConsole m_debugConsole;
+            bool m_initialized = false;
         };
     } // namespace LYIntegration
 } // namespace AZ

Gems/ImGui/Code/Include/ImGuiBus.h

@@ -90,6 +90,8 @@ namespace ImGui
         virtual void SetResolutionMode(ImGuiResolutionMode state) = 0;
         virtual const ImVec2& GetImGuiRenderResolution() const = 0;
         virtual void SetImGuiRenderResolution(const ImVec2& res) = 0;
+        virtual void OverrideRenderWindowSize(uint32_t width, uint32_t height) = 0;
+        virtual void RestoreRenderWindowSizeToDefault() = 0;
         virtual void Render() = 0;
     };
     typedef AZ::EBus<IImGuiManagerListener> ImGuiManagerListenerBus;

Gems/ImGui/Code/Source/ImGuiManager.cpp

@@ -262,6 +262,21 @@ void ImGuiManager::Shutdown()
     ImGui::DestroyContext(m_imguiContext);
 }
 
+void ImGui::ImGuiManager::OverrideRenderWindowSize(uint32_t width, uint32_t height)
+{
+    m_windowSize.m_width = width;
+    m_windowSize.m_height = height;
+    m_overridingWindowSize = true;
+    // Don't listen for window updates if our window size is being overridden
+    AzFramework::WindowNotificationBus::Handler::BusDisconnect();
+}
+
+void ImGui::ImGuiManager::RestoreRenderWindowSizeToDefault()
+{
+    m_overridingWindowSize = false;
+    InitWindowSize();
+}
+
 void ImGuiManager::Render()
 {
     if (m_clientMenuBarState == DisplayState::Hidden && m_editorWindowState == DisplayState::Hidden)
@@ -757,7 +772,7 @@ void ImGuiManager::InitWindowSize()
 {
     // We only need to initialize the window size by querying the window the first time.
     // After that we will get OnWindowResize notifications
-    if (!AzFramework::WindowNotificationBus::Handler::BusIsConnected())
+    if (!m_overridingWindowSize && !AzFramework::WindowNotificationBus::Handler::BusIsConnected())
     {
         AzFramework::NativeWindowHandle windowHandle = nullptr;
         AzFramework::WindowSystemRequestBus::BroadcastResult(windowHandle, &AzFramework::WindowSystemRequestBus::Events::GetDefaultWindowHandle);

Gems/ImGui/Code/Source/ImGuiManager.h

@@ -60,6 +60,8 @@ namespace ImGui
         void SetResolutionMode(ImGuiResolutionMode mode) override { m_resolutionMode = mode; }
         const ImVec2& GetImGuiRenderResolution() const override { return m_renderResolution; }
         void SetImGuiRenderResolution(const ImVec2& res) override { m_renderResolution = res; }
+        void OverrideRenderWindowSize(uint32_t width, uint32_t height) override;
+        void RestoreRenderWindowSizeToDefault() override;
         void Render() override;
         // -- ImGuiManagerListenerBus Interface -------------------------------------------------------------------
 
@@ -89,6 +91,7 @@ namespace ImGui
         ImVec2 m_renderResolution = ImVec2(1920.0f, 1080.0f);
         ImVec2 m_lastRenderResolution;
         AzFramework::WindowSize m_windowSize = AzFramework::WindowSize(1920, 1080);
+        bool m_overridingWindowSize = false;
 
         // Rendering buffers
         std::vector<SVF_P3F_C4B_T2F> m_vertBuffer;