Child-menus no longer attempt to fit inside the viewport

[ZingBallyhoo]

Version/Branch of Dear ImGui:

Version 1.90.2 WIP (19016), Branch: master

Back-ends:

imgui_impl_win32.cpp + imgui_impl_dx11.cpp

Compiler, OS:

Windows 10 + MSVC

Full config/build information:

Dear ImGui 1.90.2 WIP (19016)
--------------------------------
sizeof(size_t): 8, sizeof(ImDrawIdx): 2, sizeof(ImDrawVert): 20
define: __cplusplus=199711
define: _WIN32
define: _WIN64
define: _MSC_VER=1938
define: _MSVC_LANG=201402
--------------------------------
io.BackendPlatformName: imgui_impl_win32
io.BackendRendererName: imgui_impl_dx11
io.ConfigFlags: 0x00000003
 NavEnableKeyboard
 NavEnableGamepad
io.ConfigInputTextCursorBlink
io.ConfigWindowsResizeFromEdges
io.ConfigMemoryCompactTimer = 60.0
io.BackendFlags: 0x0000000E
 HasMouseCursors
 HasSetMousePos
 RendererHasVtxOffset
--------------------------------
io.Fonts: 1 fonts, Flags: 0x00000000, TexSize: 512,64
io.DisplaySize: 1264.00,761.00
io.DisplayFramebufferScale: 1.00,1.00
--------------------------------
style.WindowPadding: 8.00,8.00
style.WindowBorderSize: 1.00
style.FramePadding: 4.00,3.00
style.FrameRounding: 0.00
style.FrameBorderSize: 0.00
style.ItemSpacing: 8.00,4.00
style.ItemInnerSpacing: 4.00,4.00

Details:

My Issue/Question:

As of 1.90.1 child menus no longer attempt to fit inside of the viewport. Root menus still work correctly.

Introduced in commit f37f6f6 (Related issue: #7063)

Screenshots/Video:

Previous behavior	Current behavior

Minimal, Complete and Verifiable Example code:

if (ImGui::BeginMainMenuBar())
{
    if (ImGui::BeginMenu("Menu"))
    {
        if (ImGui::BeginMenu("Submenu"))
        {
            for (int i = 0; i < 50; ++i)
            {
                ImGui::Text("hello!");
            }
            ImGui::EndMenu();
        }
        ImGui::EndMenu();
    }
    ImGui::EndMainMenuBar();
}

Can also be seen on the demo submenus (e.g Menu->Colors)

[ZingBallyhoo]

Was this mis-triaged as multi-viewports? I feel like my title could have been misleading. What I really meant was "child-menus no longer attempt to fit inside the work rect of the viewport"

My very temporary fix is this: (master only)

Index: imgui.cpp
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/imgui.cpp b/imgui.cpp
--- a/imgui.cpp	(revision 536090303a8fca7d896f77d6d63dc59249bc87f4)
+++ b/imgui.cpp	(date 1707564068063)
@@ -5755,7 +5755,8 @@
     {
         // Maximum window size is determined by the viewport size or monitor size
         ImVec2 size_min = CalcWindowMinSize(window);
-        ImVec2 size_max = (window->Flags & ImGuiWindowFlags_ChildWindow) ? ImVec2(FLT_MAX, FLT_MAX) : ImGui::GetMainViewport()->WorkSize - style.DisplaySafeAreaPadding * 2.0f;
+        ImVec2 size_max = (window->Flags & ImGuiWindowFlags_ChildWindow && !(window->Flags & ImGuiWindowFlags_ChildMenu)) ? 
+            ImVec2(FLT_MAX, FLT_MAX) : ImGui::GetMainViewport()->WorkSize - style.DisplaySafeAreaPadding * 2.0f;
         ImVec2 size_auto_fit = ImClamp(size_desired, size_min, size_max);
 
         // When the window cannot fit all contents (either because of constraints, either because screen is too small),

[ocornut]

You are right, I've pushed the fix 3af739a.
The right fix is to check for the _Popup flag rather than _ChildMenu one.
Alternative repro being:

if (ImGui::Button("OpenPopup"))
    ImGui::OpenPopup("Popup");
if (ImGui::BeginPopup("Popup", ImGuiWindowFlags_ChildWindow))
{
    for (int i = 0; i < 50; ++i)
    {
        ImGui::Text("hello!");
    }
    ImGui::EndPopup();
}

Using Popup+Child flags together is not well documented behavior but as various uses.