Multiline text with ReadOnly can render incorrectly when text changes

[bgribble]

Version/Branch of Dear ImGui:

Version 1.91.5, via imgui_bundle

Back-ends:

SDL2

Compiler, OS:

Linux + python and c++

Full config/build information:

No response

Details:

I discovered this issue while working with ImGui Bundle in python. The full report with reproduction code in Python and C++, and a screencast of the behavior, is in pthom/imgui_bundle#295

My application uses an InputTextMultiline with the ReadOnly flag to implement an interactive REPL in the application. I made it ReadOnly so I can handle input myself, to implement app-specific keybindings for history navigation and editing.

I observed that under some circumstances the InputTextMultiline would display spaces or nonprinting characters (appearing as ???) rather than the text that was passed to the widget. Clicking in a different Imgui window in the app would cause the text to appear as expected.

I discussed with Pascal Thomet and he believes that this might be a bug in Imgui, since the following simple C++ code demonstrates it.

Screenshots/Video:

v.mp4

Minimal, Complete and Verifiable Example code:

#include <imgui.h>
#include "misc/cpp/imgui_stdlib.h"
#include <hello_imgui/hello_imgui.h>

std::string static_text = R"(
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod
tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim
veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex
ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate
velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint
occaecat cupidatat non proident, sunt in culpa qui officia deserunt
mollit anim id est laborum.
)";


char buffer[2048];

int frame_count = 0;

void gui()
{
    ImGui::Text("Should reveal text over time.");
    ImGui::Text("Click in the multiline text widget to see invisible and erroneous characters");
    ImGui::Text("Then click outside the widget to see the correct text restored");
    ImGui::Text("Code taken from demo_widgets.py");

    auto callback = [](ImGuiInputTextCallbackData* cb_data) -> int
    {
        int bufpos = frame_count / 2;
        cb_data->SelectionStart = bufpos;
        cb_data->SelectionEnd = bufpos;
        cb_data->CursorPos = bufpos;
        return 0;
    };

    memset(buffer, 0, sizeof(buffer));
    strncpy(buffer, static_text.c_str(), frame_count / 2);

    ImGui::InputTextMultiline(
        "readonly_multiline",
        buffer,
        sizeof(buffer),
        ImVec2(600, 200),
        ImGuiInputTextFlags_ReadOnly | ImGuiInputTextFlags_CallbackAlways,
        callback
    );
    frame_count++;
    if (frame_count > 2 * static_text.size())
        frame_count = 0;
}


int main()
{
    HelloImGui::Run(gui);
}

[ocornut]

Thank you for your report and the repro, @bgribble and @pthom.

I have pushed a first fix: e900571 which uses a bit of a "live dangerously" approach, and I then extended and improved that work and removed the "live dangerously" bit by introducing an internal TextSrc pointer in 32f1140 and ensuring all read only operations used it.

Even though all tests are passing, statistically speaking I won't be surprised if this introduced a small issue somewhere. We will find out.

[pthom]

Merci beaucoup Omar for your correction in a record time!

I like to live dangerously sometimes also :-)
So, since you merged the correction to the docking branch, I applied the changes to Dear ImGui Bundle in this commit, and the issue is also solved in Python!

@bgribble : the Python wheels will be available shortly here: https://github.com/pthom/imgui_bundle/actions/runs/12398172321

[ocornut]

My apologies, 32f1140 pushed on wednesday badly broke clipboard cut/copy.
Pushed fix 6982ce4

[pthom]

Thanks for letting me know!

[pthom]

I updated the python side too. Thanks !