SetNextWindowSize() on single axis doesn't perform auto-fit calculation correctly

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Version 1.92.2b

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I think window's size calculation should take account of whether the window uses scrollbar. Take the screenshots for example, both auto-resized windows (ImGuiWindowFlags_AlwaysAutoResize) and resizable windows (auto-fitting) are affected by this.

Screenshots/Video:

Auto-resized window: (| of the longest string becomes hidden by the scrollbar)
Resizable window: (| of the longest string becomes hidden after auto-fitting)

Minimal, Complete and Verifiable Example code:

static void issue() {
    static bool auto_resize = true;
    static bool fix_height = false; // * auto_resize -> scrollbar.
    if (fix_height) {
        ImGui::SetNextWindowSize({ 0,ImGui::GetTextLineHeightWithSpacing() * 10 });
    }
    if (ImGui::Begin("Issue", 0, (auto_resize ? ImGuiWindowFlags_AlwaysAutoResize : ImGuiWindowFlags_None) | ImGuiWindowFlags_NoSavedSettings)) {
        ImGui::Checkbox("AutoResize", &auto_resize);
        ImGui::Checkbox("FixHeight", &fix_height);
        for (const int n : {1, 5, 2, 3, 9, 6, 3, 1}) {
            ImGui::TextUnformatted((std::string(n + 10, 'a') + "|").c_str());
        }
    }
    ImGui::End();
}

Window-size calculation should take account of scrollbar[/-]

In most cases it does, but it seems like ImGui::SetNextWindowSize({ 0,ImGui::GetTextLineHeightWithSpacing() * 10 }); actually breaks the logic for auto-fitting indeed.

Note that this works:

if (fix_height)
{
    float height = ImGui::GetTextLineHeightWithSpacing() * 10;
--  ImGui::SetNextWindowSize({ 0, height });
++  ImGui::SetNextWindowSizeConstraints(ImVec2(0.0f, height), ImVec2(FLT_MAX, height));
}

Thanks! Changing SetNextWindowSize to SetNextWindowSizeConstraints solves my original problem.

So the second screenshot (auto-fitting) seems a separate problem. Initially I thought they are the same one.

This is enough to reproduce the second screenshot. (When the window has vertical scrollbar and auto-fit horizontally.)

{
    if (ImGui::Begin("Auto", 0, ImGuiWindowFlags_NoSavedSettings)) {
        for (const int n : {1, 5, 2, 3, 9, 6, 3, 1}) {
            ImGui::TextUnformatted((std::string(n + 10, 'a') + "|").c_str());
        }
    }
    ImGui::End();
}

Right, this correspond to this FIXME in UpdateWindowManualResize():

// Double-clicking bottom or right border auto-fit on this axis
// FIXME: CalcWindowAutoFitSize() doesn't take into account that only one side may be auto-fit when calculating scrollbars.
if (border_n == 1 || border_n == 3) // Right and bottom border
{
    size_target[axis] = CalcWindowSizeAfterConstraint(window, size_auto_fit)[axis];
    ret_auto_fit_mask |= (1 << axis);
    hovered = held = false; // So border doesn't show highlighted at new position
}

Since both uses of size_auto_fit are on specific actions, we could remove this parameter and instead redesign CalcWindowAutoFitSize() to support single-axis mode, and call this in UpdateWindowManualResize().

I believe this should fix the second issue, and it's passing imgui_test_suite, and can be foundation to fix the first issue too.

imgui-7fe39d9-WIP fix 9060.patch

But I'll work on adding more tests first.

Windows: fixed single-axis auto-sizing (via double-clicking a border)…

Added test : ocornut/imgui_test_engine@16442d5
Pushed fix for border resizing (second issue): 7537ba2

Note that 7537ba2 actually removed the need for the hack added in 29439bd and tested by "window_child_resize". (#1710)

Windows: reorganize auto-fitting code blocks in Begin(), aimed to hav…

Windows: reorganize auto-fitting code blocks in Begin(), step 2.

Windows: programmatic auto-sizing on a single axis also apply proper …

Windows: CalcWindowAutoFitSize() remove child-specific hack added by 2…

Both cases should be fixed now.
Because window sizing code is extremely sensitive I zealously split the changes in many commits.

It's not actually many changes but out of experience this is generally going to be useful if I need to come back to this later.

Now bracing for impact....

Windows: fixed single-axis auto-sizing (via double-clicking a border)…

Windows: reorganize auto-fitting code blocks in Begin(), aimed to hav…

Windows: reorganize auto-fitting code blocks in Begin(), step 2.

Windows: programmatic auto-sizing on a single axis also apply proper …

Windows: CalcWindowAutoFitSize() remove child-specific hack added by 2…

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SetNextWindowSize() on single axis doesn't perform auto-fit calculation correctly #9060

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SetNextWindowSize() on single axis doesn't perform auto-fit calculation correctly #9060

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ocornut commented on Nov 10, 2025

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ocornut commented on Nov 11, 2025

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