Example code for reproducing thedmd/imgui-node-editor#205

Build instructions

./fetch_external.sh
mkdir build
cd build
cmake ..
make -j

Code explanation

It was not possible to use the ImGui version inside node editor, since it is outdated, hence this separate repository.

The code is contained in two short files:

main_imgui_sdl3_opengl3.cpp: This file is an almost direct copy of imgui/examples/example_glfw_opengl3/main.cpp, the only difference being that it calls functions from node_clipping_issue.cpp (for the Gui and context setup)
node_clipping_issue.cpp Demo code.

Issues explanations

This demo is able to trigger subtle issues that arise when using docked windows, and/or windows whose size is unknown at the first frame (so that the canvas does not know its size)

4 issues can be triggered: they are detailed below. I'm sorry if all of them are grouped in a single project, but it seemed to be the most efficient way to communicate about them. Most of the fixes are simple, but the fix for issue 205 is only a workaround.

Issue 205: thedmd/imgui-node-editor#205

Clipping is not related to the window with the latest imgui"

Adding a simple pixel at m_WidgetRect.Max is enough to bypass the issue. This is not a real fix, since it merely adds a fake pixel at (xmax, ymax), so that the app knows where the clipping should end. A better solution would be required.

Workaround: at imgui_canvas.cpp:122 (see commit)

bool ImGuiEx::Canvas::Begin(ImGuiID id, const ImVec2& size)
{
    ...
    ...
   // Adding a simple pixel at m_WidgetRect.Max is enough to temporarily solve the issue
   // (the color is selected to be as transparent as possible)
   m_DrawList->AddLine(m_WidgetRect.Max, m_WidgetRect.Max, IM_COL32(0, 0, 0, 1)); // <= **** Added this workaround ****

PR "186" : thedmd/imgui-node-editor#186

if ImGuiEx::Canvas::Begin returns false (which can happen if the window is too small and the canvas is Clipped), then the subsequent call to ImGuiEx::Canvas::End() will trigger an IM_ASSERT

This bug is quite complex to trigger:

the window size must be still unknown at first display
you may need to remove imgui.ini and NodeEditor.json to trigger it so that the window size is actually unknown
a widget must be displayed before the node editor (see ImGui::Text("Hello world") below)

Fix: at imgui_node_editor.cpp:1126 (see commit)*

void ed::EditorContext::Begin(const char* id, const ImVec2& size)
{
...

         if (!m_IsInitialized)
         {
            // Cycle canvas so it has a change to setup its size before settings are loaded
            if (m_Canvas.Begin(id, canvasSize))     // <== **** Added this if ****
                m_Canvas.End();
            LoadSettings();
            m_IsInitialized = true;
         }

Issue with bool ed::CreateItemAction::Begin() (not mentioned in the repo issues yet)

ed::CreateItemAction::Begin: if (m_CurrentStage == None) => m_InActive = false Otherwise, we may reach IM_ASSERT(false == m_InActive) upon next call. This happens at first display

Fix: at imgui_node_editor.cpp:4710 (see commit)

bool ed::CreateItemAction::Begin()
{
    ...
    ...
    if (m_CurrentStage == None)
    {
        m_InActive = false;       // <== **** Added this statement ****
        return false;
    }

remove duplicate ImVec2 operator-

(operator- is now provided by imgui)

pthom / node_window_clipping_issue