LeaveLocalSpace(): Search for ImDrawCallback_ImCanvas after m_DrawListFirstCommandIndex (proposed fix for thedmd#282)

        Analysis of the bug thedmd#282
        when using this repro: https://github.com/pthom/node_window_clipping_issue/tree/suspend_resume_issue2

        At the 4th call of LeaveLocalSpace(), we have:

            this = {ImGuiEx::Canvas *}
                 m_DrawList = {ImDrawList *}                      mDrawList is of size 4
                     CmdBuffer = {ImVector<ImDrawCmd>}            its elements at index 2
                         Size = {int} 4                           has UserCallback == {ImDrawCallback_ImCanvas}
                         Capacity = {int} 8
                         Data = {ImDrawCmd *}
                 m_ExpectedChannel = {int} 0
                 m_DrawListFirstCommandIndex = {int} 2
                 m_DrawListCommadBufferSize = {int} 1
                 m_DrawListStartVertexIndex = {int} 8

         We have m_DrawListFirstCommandIndex = 2, however, the tests in the original code will test only at index 0 and 1!
                if (m_DrawList->CmdBuffer.size() > m_DrawListCommadBufferSize && m_DrawList->CmdBuffer[m_DrawListCommadBufferSize].UserCallback == ImDrawCallback_ImCanvas)
                    m_DrawList->CmdBuffer.erase(m_DrawList->CmdBuffer.Data + m_DrawListCommadBufferSize);
                else if (m_DrawList->CmdBuffer.size() >= m_DrawListCommadBufferSize && m_DrawList->CmdBuffer[m_DrawListCommadBufferSize - 1].UserCallback == ImDrawCallback_ImCanvas)
                    m_DrawList->CmdBuffer.erase(m_DrawList->CmdBuffer.Data + m_DrawListCommadBufferSize - 1);

        // Proposed solution: test all commands from index >= m_DrawListFirstCommandIndex
        // and remove the one with UserCallback == ImDrawCallback_ImCanvas
        // (based on the original code, it seems there can be only one)
        int idxCommand_ImDrawCallback_ImCanvas = -1;
        for (int i = m_DrawListFirstCommandIndex; i < m_DrawList->CmdBuffer.size(); ++i)
        {
            auto & command = m_DrawList->CmdBuffer[i];
            if (command.UserCallback == ImDrawCallback_ImCanvas)
            {
                idxCommand_ImDrawCallback_ImCanvas = i;
                break;
            }
        }
        if (idxCommand_ImDrawCallback_ImCanvas >= 0)
            m_DrawList->CmdBuffer.erase(m_DrawList->CmdBuffer.Data + idxCommand_ImDrawCallback_ImCanvas);

Author: pthom

Diff for: imgui_canvas.cpp

@@ -557,10 +557,53 @@ void ImGuiEx::Canvas::LeaveLocalSpace()
     // Remove sentinel draw command if present
     if (m_DrawListCommadBufferSize > 0)
     {
+        /*
+        Analysis of the bug https://github.com/thedmd/imgui-node-editor/issues/282
+        when using this repro: https://github.com/pthom/node_window_clipping_issue/tree/suspend_resume_issue2
+
+        At the 4th call of LeaveLocalSpace(), we have:
+
+            this = {ImGuiEx::Canvas *}
+                 m_DrawList = {ImDrawList *}                      mDrawList is of size 4
+                     CmdBuffer = {ImVector<ImDrawCmd>}            its elements at index 2
+                         Size = {int} 4                           has UserCallback == {ImDrawCallback_ImCanvas}
+                         Capacity = {int} 8
+                         Data = {ImDrawCmd *}
+                 m_ExpectedChannel = {int} 0
+                 m_DrawListFirstCommandIndex = {int} 2
+                 m_DrawListCommadBufferSize = {int} 1
+                 m_DrawListStartVertexIndex = {int} 8
+
+         We have m_DrawListFirstCommandIndex = 2, however, the tests in the original code will test only at index 0 and 1!
+                if (m_DrawList->CmdBuffer.size() > m_DrawListCommadBufferSize && m_DrawList->CmdBuffer[m_DrawListCommadBufferSize].UserCallback == ImDrawCallback_ImCanvas)
+                    m_DrawList->CmdBuffer.erase(m_DrawList->CmdBuffer.Data + m_DrawListCommadBufferSize);
+                else if (m_DrawList->CmdBuffer.size() >= m_DrawListCommadBufferSize && m_DrawList->CmdBuffer[m_DrawListCommadBufferSize - 1].UserCallback == ImDrawCallback_ImCanvas)
+                    m_DrawList->CmdBuffer.erase(m_DrawList->CmdBuffer.Data + m_DrawListCommadBufferSize - 1);
+
+
+        */
+
+        // Original tests; this test will fail because it tests at index 0 and 1
         if (m_DrawList->CmdBuffer.size() > m_DrawListCommadBufferSize && m_DrawList->CmdBuffer[m_DrawListCommadBufferSize].UserCallback == ImDrawCallback_ImCanvas)
             m_DrawList->CmdBuffer.erase(m_DrawList->CmdBuffer.Data + m_DrawListCommadBufferSize);
         else if (m_DrawList->CmdBuffer.size() >= m_DrawListCommadBufferSize && m_DrawList->CmdBuffer[m_DrawListCommadBufferSize - 1].UserCallback == ImDrawCallback_ImCanvas)
             m_DrawList->CmdBuffer.erase(m_DrawList->CmdBuffer.Data + m_DrawListCommadBufferSize - 1);
+
+        // Proposed solution: test all commands from index >= m_DrawListFirstCommandIndex
+        // and remove the one with UserCallback == ImDrawCallback_ImCanvas
+        // (based on the original code, it seems there can be only one)
+        int idxCommand_ImDrawCallback_ImCanvas = -1;
+        for (int i = m_DrawListFirstCommandIndex; i < m_DrawList->CmdBuffer.size(); ++i)
+        {
+            auto & command = m_DrawList->CmdBuffer[i];
+            if (command.UserCallback == ImDrawCallback_ImCanvas)
+            {
+                idxCommand_ImDrawCallback_ImCanvas = i;
+                break;
+            }
+        }
+        if (idxCommand_ImDrawCallback_ImCanvas >= 0)
+            m_DrawList->CmdBuffer.erase(m_DrawList->CmdBuffer.Data + idxCommand_ImDrawCallback_ImCanvas);
     }
 
     auto& fringeScale = ImFringeScaleRef(m_DrawList);