Incorrect appearing size caused by child windows

[achabense]

Version/Branch of Dear ImGui:

Version 1.92.2b

Back-ends:

imgui_impl_sdl3.cpp + imgui_impl_sdlrenderer3.cpp

Compiler, OS:

MSVC

Full config/build information:

No response

Details:

If an auto-resized window contains another child window with ImGuiChildFlags_AutoResize.. flags, the window will have wrong size at the first visible frame. (See the example code for more context.)

Workarounds: 1. Calculate content size manually (but that beats the purpose of auto-resize). 2. Hide the window for one extra frame.

Screenshots/Video:

Minimal, Complete and Verifiable Example code:

static void issue() {
    if (ImGui::Begin("Issue")) {
        static bool slow = false;
        static int number = 0;
        ImGui::Checkbox("Slow", &slow);
        if (slow) {
            Sleep(100);
        }
        if (ImGui::Button("Popup")) {
            ImGui::OpenPopup("Example");
            number = rand() % 5; // 0 ~ 4
        }
        if (ImGui::BeginPopup("Example")) {
            ImGui::Button("##other contents", { 200,60 });
            // I want to achieve the following effects:
            // 1. The actual required width is not known in advance (e.g. list of files), and can be calculated automatically.
            // 2. The height is dependent on number of items, but I can easily specify height limit.
            // 3. If the contents reach the height limit, the window will show a scrollbar.

            // The following combination works well, but will have wrong size (only) at first visible frame, causing a small flick.
            // Intuitively, this can be avoided as the popup has already hidden itself for one frame for size calculation.
            // (Turn on 'Slow' to see the effect.)
            if (number != 0) {
                ImGui::SetNextWindowSizeConstraints({ 0,0 }, { 300,100 });
                if (ImGui::BeginChild("List", { 0,0 }, ImGuiChildFlags_AutoResizeX | ImGuiChildFlags_AutoResizeY)) {
                    for (int i = 0; i < number; ++i) {
                        ImGui::PushID(i);
                        ImGui::Button("##Item", { 220,30 });
                        ImGui::PopID();
                    }
                }
                ImGui::EndChild();
            }

            ImGui::EndPopup();
        }
    }
    ImGui::End();
}

[ocornut]

This is known and has always been the case. Any auto-resizing requires one frame to proceed.
There's no way around it.

If you wish to hide the parent you can do, e.g.

                [....]
                ImGuiWindow* child_window = ImGui::GetCurrentWindow();
                child_hidden = child_window->Hidden;
                ImGui::EndChild();
            }
            if (child_hidden)
            {
                ImGuiWindow* window = ImGui::GetCurrentWindow();
                window->HiddenFramesForRenderOnly = 2;
            }

But note that isn't specific to child windows, and it's not even generally "correct" to use "child_window->Hidden" as a source of information. It's just an acceptable proxy here since you know that child size is locked at each opening (since you compute number = rand() % 5; // 0 ~ 4 next to OpenPopup()). If you were to resize contents in the parent after the appearing frame it would be one frame late as well.

[achabense]

Thanks! Glad to know about HiddenFramesForRenderOnly.

[ocornut]

Closing this as I don't think there's much more to add to this now.