Editing the buffer in InputTextCallback desyncs the undo/redo stack

[JoshuaWebb]

Version/Branch of Dear ImGui:

Version: 1.87
Branch: master

Back-end/Renderer/Compiler/OS

Back-ends: imgui_impl_win32.cpp + imgui_impl_dx11.cpp (although it doesn't actually matter for this bug)
Operating System: Windows 10

My Issue/Question:

Editing an InputText via ImGuiInputTextCallbackData doesn't register any entries in the undo/redo stack.

Screenshots/Video

Repro steps:

1. Using the provided sample placed into one of the example main.cpp
2. Type some characters into Input
3. Optionally type some characters into Replacement
4. Make a selection in Input to replace
5. Press Ctrl+R to trigger the replacement callback
6. Press Ctrl+Z (Undo) once. The input text is now incorrect, and the cursor is wrong.
7. Press Ctrl+Y (Redo) once. The application crashes with an assertion (Assertion failed: pos <= text_len, file ..\..\imgui_widgets.cpp, line 3667)

Standalone, minimal, complete and verifiable example:

struct MyData {
    char InputBuf[256];
    char Replacement[256];

    MyData() { memset(this, 0, sizeof(*this)); }
};
int TextEditCallback(ImGuiInputTextCallbackData* data)
{
    ImGuiIO& io = ImGui::GetIO();

    MyData *my_data = (MyData*)data->UserData;

    switch (data->EventFlag)
    {
    case ImGuiInputTextFlags_CallbackAlways:
        {
            if (io.KeyCtrl && ImGui::IsKeyPressed('R')) {

                int pos = data->SelectionStart < data->SelectionEnd
                          ? data->SelectionStart
                          : data->SelectionEnd;
                int del_count = abs(data->SelectionEnd - data->SelectionStart);
                if (del_count > 0) {
                    data->DeleteChars(pos, del_count);
                }

                data->InsertChars(pos, my_data->Replacement);
            }

            break;
        }
    }
    return 0;
}

int main(int, char**)
{
        // .... 

        // 4. Repro
        {
            static MyData my_data;
            ImGui::SetNextWindowSize(ImVec2(380,120), ImGuiCond_Appearing);
            ImGui::Begin("Example");
            ImGui::InputText("Replacement", my_data.Replacement, IM_ARRAYSIZE(my_data.Replacement));
            ImGui::InputText("Input", my_data.InputBuf, IM_ARRAYSIZE(my_data.InputBuf), ImGuiInputTextFlags_CallbackAlways, &TextEditCallback, &my_data);
            ImGui::End();
        }

        // Rendering
        // ...
}

[JoshuaWebb]

I've added a pull request (#4949) with the code I've used to fix this problem for myself in case that's useful.

[nukeulater]

there's another way to trigger the stack desync.
here's an example from the demo console that's provided with ImGui

1. type and execute a command (e.g. clear)
2. hold Ctrl+Z (notice how the cursor moves back to the last character position of the command)
3. try to type something, assert() gets triggered

[ocornut]

Thank you @JoshuaWebb and @nukeulater for those details and PR, will try to look soon.

[ocornut]

I have pushed a first fix for the second issue (submitted by @nukeulater) which is a different one than the first one (fixed by #4949). The fix include a new tool in Metrics to visualize the undo state:

[ocornut]

Fixed with 530332d + amends a35e876