Updated to Unreal Engine 4.17 (backward compatible):

- Removed usage of per-vertex ClipRect.
- Replaced usage of GSlateScissorRect with new FSlateWindowElementList::PushClip/PopClip functions.
- Defined WITH_OBSOLETE_CLIPPING_API to switch to old implementation for older engine versions.

Source/ImGui/Private/ImGuiDrawData.cpp

@@ -5,7 +5,11 @@
 #include "ImGuiDrawData.h"
 
 
+#if WITH_OBSOLETE_CLIPPING_API
 void FImGuiDrawList::CopyVertexData(TArray<FSlateVertex>& OutVertexBuffer, const FVector2D VertexPositionOffset, const FSlateRotatedRect& VertexClippingRect) const
+#else
+void FImGuiDrawList::CopyVertexData(TArray<FSlateVertex>& OutVertexBuffer, const FVector2D VertexPositionOffset) const
+#endif // WITH_OBSOLETE_CLIPPING_API
 {
 	// Reset and reserve space in destination buffer.
 	OutVertexBuffer.SetNumUninitialized(ImGuiVertexBuffer.Size, false);
@@ -25,8 +29,10 @@ void FImGuiDrawList::CopyVertexData(TArray<FSlateVertex>& OutVertexBuffer, const
 		SlateVertex.Position[0] = ImGuiVertex.pos.x + VertexPositionOffset.X;
 		SlateVertex.Position[1] = ImGuiVertex.pos.y + VertexPositionOffset.Y;
 
+#if WITH_OBSOLETE_CLIPPING_API
 		// Set clipping rectangle.
 		SlateVertex.ClipRect = VertexClippingRect;
+#endif // WITH_OBSOLETE_CLIPPING_API
 
 		// Unpack ImU32 color.
 		SlateVertex.Color = ImGuiInterops::UnpackImU32Color(ImGuiVertex.col);

Source/ImGui/Private/ImGuiDrawData.h

@@ -4,11 +4,16 @@
 
 #include "ImGuiInteroperability.h"
 
+#include <Runtime/Launch/Resources/Version.h>
 #include <SlateCore.h>
 
 #include <imgui.h>
 
 
+// Starting from version 4.17 Slate doesn't have per-vertex clipping rectangle and GSlateScissorRect. Use this to
+// support older engine versions.
+#define WITH_OBSOLETE_CLIPPING_API		(ENGINE_MAJOR_VERSION < 4 || (ENGINE_MAJOR_VERSION == 4 && ENGINE_MINOR_VERSION < 17))
+
 // ImGui draw command data transformed for Slate.
 struct FImGuiDrawCommand
 {
@@ -34,11 +39,18 @@ public:
 		return{ ImGuiCommand.ElemCount, ImGuiInterops::ToSlateRect(ImGuiCommand.ClipRect), ImGuiInterops::ToTextureIndex(ImGuiCommand.TextureId) };
 	}
 
+#if WITH_OBSOLETE_CLIPPING_API
 	// Transform and copy vertex data to target buffer (old data in the target buffer are replaced).
 	// @param OutVertexBuffer - Destination buffer
 	// @param VertexPositionOffset - Position offset added to every vertex to transform it to different space
 	// @param VertexClippingRect - Clipping rectangle for Slate vertices
 	void CopyVertexData(TArray<FSlateVertex>& OutVertexBuffer, const FVector2D VertexPositionOffset, const FSlateRotatedRect& VertexClippingRect) const;
+#else
+	// Transform and copy vertex data to target buffer (old data in the target buffer are replaced).
+	// @param OutVertexBuffer - Destination buffer
+	// @param VertexPositionOffset - Position offset added to every vertex to transform it to different space
+	void CopyVertexData(TArray<FSlateVertex>& OutVertexBuffer, const FVector2D VertexPositionOffset) const;
+#endif // WITH_OBSOLETE_CLIPPING_API
 
 	// Transform and copy index data to target buffer (old data in the target buffer are replaced).
 	// Internal index buffer contains enough data to match the sum of NumElements from all draw commands.

Source/ImGui/Private/SImGuiWidget.cpp

@@ -403,12 +403,15 @@ int32 SImGuiWidget::OnPaint(const FPaintArgs& Args, const FGeometry& AllottedGeo
 
 		for (const auto& DrawList : ContextProxy->GetDrawData())
 		{
+#if WITH_OBSOLETE_CLIPPING_API
 			DrawList.CopyVertexData(VertexBuffer, VertexPositionOffset, VertexClippingRect);
 
 			// Get access to the Slate scissor rectangle defined in Slate Core API, so we can customize elements drawing.
 			extern SLATECORE_API TOptional<FShortRect> GSlateScissorRect;
-
 			auto GSlateScissorRectSaver = ScopeGuards::MakeStateSaver(GSlateScissorRect);
+#else
+			DrawList.CopyVertexData(VertexBuffer, VertexPositionOffset);
+#endif // WITH_OBSOLETE_CLIPPING_API
 
 			int IndexBufferOffset = 0;
 			for (int CommandNb = 0; CommandNb < DrawList.NumCommands(); CommandNb++)
@@ -423,11 +426,21 @@ int32 SImGuiWidget::OnPaint(const FPaintArgs& Args, const FGeometry& AllottedGeo
 				// Get texture resource handle for this draw command (null index will be also mapped to a valid texture).
 				const FSlateResourceHandle& Handle = ModuleManager->GetTextureManager().GetTextureHandle(DrawCommand.TextureId);
 
-				// Transform clipping rectangle to screen space and set in Slate, to apply it to elements that we draw.
+				// Transform clipping rectangle to screen space and apply to elements that we draw.
-				GSlateScissorRect = FShortRect{ DrawCommand.ClippingRect.OffsetBy(MyClippingRect.GetTopLeft()).IntersectionWith(MyClippingRect) };
+				const FSlateRect ClippingRect = DrawCommand.ClippingRect.OffsetBy(MyClippingRect.GetTopLeft()).IntersectionWith(MyClippingRect);
+
+#if WITH_OBSOLETE_CLIPPING_API
+				GSlateScissorRect = FShortRect{ ClippingRect };
+#else
+				OutDrawElements.PushClip(FSlateClippingZone{ ClippingRect });
+#endif // WITH_OBSOLETE_CLIPPING_API
 
 				// Add elements to the list.
 				FSlateDrawElement::MakeCustomVerts(OutDrawElements, LayerId, Handle, VertexBuffer, IndexBuffer, nullptr, 0, 0);
+
+#if !WITH_OBSOLETE_CLIPPING_API
+				OutDrawElements.PopClip();
+#endif // WITH_OBSOLETE_CLIPPING_API
 			}
 		}
 	}