Fix ImGui rendering in-Editor

-Ensure ViewportContext rendertick notifications always fire
-Use viewport size to determine ImGui resolution, ensure OnViewportSizeChanged is always up-to-date for the default viewport context

Gems/Atom/RPI/Code/Include/Atom/RPI.Public/ViewportContext.h

@@ -77,6 +77,8 @@ namespace AZ
             void OnRenderPipelineAdded(RenderPipelinePtr pipeline) override;
             //! Ensures our default view remains set when our scene's render pipelines are modified.
             void OnRenderPipelineRemoved(RenderPipeline* pipeline) override;
+            //! OnBeginPrepareRender is forwarded to our RenderTick notification to allow subscribers to do rendering.
+            void OnBeginPrepareRender() override;
 
             //WindowNotificationBus interface
             //! Used to fire a notification when our window resizes

Gems/Atom/RPI/Code/Source/RPI.Public/ViewportContext.cpp

@@ -104,12 +104,16 @@ namespace AZ
             // add the current pipeline to next render tick if it's not already added.
             if (m_currentPipeline && m_currentPipeline->GetRenderMode() != RenderPipeline::RenderMode::RenderOnce)
             {
-                ViewportContextNotificationBus::Event(GetName(), &ViewportContextNotificationBus::Events::OnRenderTick);
-                ViewportContextIdNotificationBus::Event(GetId(), &ViewportContextIdNotificationBus::Events::OnRenderTick);
                 m_currentPipeline->AddToRenderTickOnce();
             }
         }
 
+        void ViewportContext::OnBeginPrepareRender()
+        {
+            ViewportContextNotificationBus::Event(GetName(), &ViewportContextNotificationBus::Events::OnRenderTick);
+            ViewportContextIdNotificationBus::Event(GetId(), &ViewportContextIdNotificationBus::Events::OnRenderTick);
+        }
+
         AZ::Name ViewportContext::GetName() const
         {
             return m_name;

Gems/Atom/RPI/Code/Source/RPI.Public/ViewportContextManager.cpp

@@ -57,9 +57,14 @@ namespace AZ
                     return;
                 }
                 viewportData.context = viewportContext;
-                auto onSizeChanged = [contextName, viewportId](AzFramework::WindowSize size)
+                auto onSizeChanged = [this, viewportId](AzFramework::WindowSize size)
                 {
-                    ViewportContextNotificationBus::Event(contextName, &ViewportContextNotificationBus::Events::OnViewportSizeChanged, size);
+                    // Ensure we emit OnViewportSizeChanged with the correct name.
+                    auto viewportContext = this->GetViewportContextById(viewportId);
+                    if (viewportContext)
+                    {
+                        ViewportContextNotificationBus::Event(viewportContext->GetName(), &ViewportContextNotificationBus::Events::OnViewportSizeChanged, size);
+                    }
                     ViewportContextIdNotificationBus::Event(viewportId, &ViewportContextIdNotificationBus::Events::OnViewportSizeChanged, size);
                 };
                 viewportContext->m_name = contextName;
@@ -174,6 +179,8 @@ namespace AZ
             GetOrCreateViewStackForContext(newContextName);
             viewportContext->m_name = newContextName;
             UpdateViewForContext(newContextName);
+            // Ensure anyone listening on per-name viewport size updates gets notified.
+            ViewportContextNotificationBus::Event(newContextName, &ViewportContextNotificationBus::Events::OnViewportSizeChanged, viewportContext->GetViewportSize());
         }
 
         void ViewportContextManager::EnumerateViewportContexts(AZStd::function<void(ViewportContextPtr)> visitorFunction)

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

@@ -58,6 +58,15 @@ namespace AZ
             auto atomViewportRequests = AZ::Interface<AZ::RPI::ViewportContextRequestsInterface>::Get();
             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
         }
 
         void ImguiAtomSystemComponent::Deactivate()
@@ -75,6 +84,13 @@ namespace AZ
         {
 #if defined(IMGUI_ENABLED)
             ImGui::ImGuiManagerListenerBus::Broadcast(&ImGui::IImGuiManagerListener::Render);
+#endif
+        }
+
+        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)});
 #endif
         }
     }

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

@@ -54,6 +54,7 @@ namespace AZ
 
             // ViewportContextNotificationBus overrides...
             void OnRenderTick() override;
+            void OnViewportSizeChanged(AzFramework::WindowSize size) override;
 
             DebugConsole m_debugConsole;
         };