Fix ImGui hover dead zones at negative canvas coordinates

Port upstream imgui-node-editor's IMGUI_HAS_VIEWPORT support into the
vendored canvas: docking-branch ImGui's IsMouseHoveringRect requires
rects to overlap the mouse viewport's screen-space rect, so items at
canvas coordinates outside the screen range (e.g. negative) never
hovered or clicked. Transform the viewport into canvas-local space in
EnterLocalSpace() and restore it in LeaveLocalSpace(), like the mouse
position. Fixes node hover highlight and in-node widget interaction
left of / above the canvas origin.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/ui/NodeEditor/Source/imgui_canvas.cpp

@@ -94,6 +94,7 @@ bool ImGuiEx::Canvas::Begin(ImGuiID id, const ImVec2& size)
 # endif
 
     SaveInputState();
+    SaveViewportState();
 
     EnterLocalSpace();
 
@@ -319,6 +320,46 @@ void ImGuiEx::Canvas::RestoreInputState()
     ImGui::GetCurrentWindow()->DC.CursorMaxPos = m_WindowCursorMaxBackup;
 }
 
+// UsdLayerManager local patch, ported from upstream imgui-node-editor
+// (see the declaration in imgui_canvas.h for why).
+void ImGuiEx::Canvas::SaveViewportState()
+{
+# if defined(IMGUI_HAS_VIEWPORT)
+    auto window = ImGui::GetCurrentWindow();
+    auto viewport = ImGui::GetWindowViewport();
+
+    m_WindowPosBackup = window->Pos;
+    m_ViewportPosBackup = viewport->Pos;
+    m_ViewportSizeBackup = viewport->Size;
+# if IMGUI_VERSION_NUM > 18002
+    m_ViewportWorkPosBackup = viewport->WorkPos;
+    m_ViewportWorkSizeBackup = viewport->WorkSize;
+# else
+    m_ViewportWorkOffsetMinBackup = viewport->WorkOffsetMin;
+    m_ViewportWorkOffsetMaxBackup = viewport->WorkOffsetMax;
+# endif
+# endif
+}
+
+void ImGuiEx::Canvas::RestoreViewportState()
+{
+# if defined(IMGUI_HAS_VIEWPORT)
+    auto window = ImGui::GetCurrentWindow();
+    auto viewport = ImGui::GetWindowViewport();
+
+    window->Pos = m_WindowPosBackup;
+    viewport->Pos = m_ViewportPosBackup;
+    viewport->Size = m_ViewportSizeBackup;
+# if IMGUI_VERSION_NUM > 18002
+    viewport->WorkPos = m_ViewportWorkPosBackup;
+    viewport->WorkSize = m_ViewportWorkSizeBackup;
+# else
+    viewport->WorkOffsetMin = m_ViewportWorkOffsetMinBackup;
+    viewport->WorkOffsetMax = m_ViewportWorkOffsetMaxBackup;
+# endif
+# endif
+}
+
 void ImGuiEx::Canvas::EnterLocalSpace()
 {
     // Prepare ImDrawList for drawing in local coordinate system:
@@ -374,6 +415,38 @@ void ImGuiEx::Canvas::EnterLocalSpace()
     for (auto i = 0; i < IM_ARRAYSIZE(m_MouseClickedPosBackup); ++i)
         io.MouseClickedPos[i] = (m_MouseClickedPosBackup[i] - m_ViewTransformPosition) * m_View.InvScale;
 
+    // UsdLayerManager local patch, ported from upstream imgui-node-editor:
+    // move the viewport rect into local space too, so IsMouseHoveringRect's
+    // screen-space viewport overlap check (docking-branch ImGui) passes for
+    // items at canvas coordinates outside the screen range (e.g. negative).
+    // See SaveViewportState() declaration for the full story.
+# if defined(IMGUI_HAS_VIEWPORT)
+    {
+        auto window = ImGui::GetCurrentWindow();
+        window->Pos = ImVec2(0.0f, 0.0f);
+
+        auto viewport_min = m_ViewportPosBackup;
+        auto viewport_max = m_ViewportPosBackup + m_ViewportSizeBackup;
+
+        viewport_min.x = (viewport_min.x - m_ViewTransformPosition.x) * m_View.InvScale;
+        viewport_min.y = (viewport_min.y - m_ViewTransformPosition.y) * m_View.InvScale;
+        viewport_max.x = (viewport_max.x - m_ViewTransformPosition.x) * m_View.InvScale;
+        viewport_max.y = (viewport_max.y - m_ViewTransformPosition.y) * m_View.InvScale;
+
+        auto viewport = ImGui::GetWindowViewport();
+        viewport->Pos  = viewport_min;
+        viewport->Size = viewport_max - viewport_min;
+
+# if IMGUI_VERSION_NUM > 18002
+        viewport->WorkPos  = m_ViewportWorkPosBackup  * m_View.InvScale;
+        viewport->WorkSize = m_ViewportWorkSizeBackup * m_View.InvScale;
+# else
+        viewport->WorkOffsetMin = m_ViewportWorkOffsetMinBackup * m_View.InvScale;
+        viewport->WorkOffsetMax = m_ViewportWorkOffsetMaxBackup * m_View.InvScale;
+# endif
+    }
+# endif
+
     m_ViewRect = CalcViewRect(m_View);;
 
     auto& fringeScale = ImFringeScaleRef(m_DrawList);
@@ -449,4 +522,5 @@ void ImGuiEx::Canvas::LeaveLocalSpace()
     ImGui::PopClipRect();
 
     RestoreInputState();
+    RestoreViewportState();
 }

src/ui/NodeEditor/Source/imgui_canvas.h

@@ -210,6 +210,18 @@ private:
     void SaveInputState();
     void RestoreInputState();
 
+    // UsdLayerManager local patch, ported from upstream imgui-node-editor:
+    // with a viewport-enabled ImGui (IMGUI_HAS_VIEWPORT, i.e. the docking
+    // branch), ImGui::IsMouseHoveringRect() — and therefore every item's
+    // hover test — requires the rect to overlap the mouse viewport's rect,
+    // which is in *screen* space. Inside the canvas, rects are in canvas
+    // space, so anything at canvas coordinates outside the screen-space
+    // range (e.g. negative) went hover/click-dead. Fix: transform the
+    // viewport rect into canvas-local space while inside EnterLocalSpace()
+    // and restore it on LeaveLocalSpace(), exactly like the mouse position.
+    void SaveViewportState();
+    void RestoreViewportState();
+
     void EnterLocalSpace();
     void LeaveLocalSpace();
 
@@ -243,6 +255,19 @@ private:
     ImVec2 m_MousePosPrevBackup;
     ImVec2 m_MouseClickedPosBackup[IM_ARRAYSIZE(ImGuiIO::MouseClickedPos)];
     ImVec2 m_WindowCursorMaxBackup;
+
+# if defined(IMGUI_HAS_VIEWPORT)
+    ImVec2 m_WindowPosBackup;
+    ImVec2 m_ViewportPosBackup;
+    ImVec2 m_ViewportSizeBackup;
+# if IMGUI_VERSION_NUM > 18002
+    ImVec2 m_ViewportWorkPosBackup;
+    ImVec2 m_ViewportWorkSizeBackup;
+# else
+    ImVec2 m_ViewportWorkOffsetMinBackup;
+    ImVec2 m_ViewportWorkOffsetMaxBackup;
+# endif
+# endif
 };
 
 } // namespace ImGuiEx