Windows, Menus: Fixed an issue where the size of sub-menu in their own viewport would be erroneously clamped to the size of main viewport. (#7730)

Amend #7287, #7063

docs/CHANGELOG.txt

@@ -93,6 +93,8 @@ Other changes:
 
 Docking+Viewports Branch:
 
+- Windows, Menus: Fixed an issue where the size of sub-menu in their own viewport
+  would be erroneously clamped to the size of main viewport. (#7730)
 - Backends: SDL3: Update for introduction of SDL_GLContext from void*. (#7701, #7702)
   [@bcsanches]
 - Backends: Win32: Secondary viewports WndProc handler retrieve/set imgui context from

imgui.cpp

@@ -6173,10 +6173,18 @@ static ImVec2 CalcWindowAutoFitSize(ImGuiWindow* window, const ImVec2& size_cont
     {
         // Maximum window size is determined by the viewport size or monitor size
         ImVec2 size_min = CalcWindowMinSize(window);
-        ImVec2 size_max = (window->ViewportOwned || ((window->Flags & ImGuiWindowFlags_ChildWindow) && !(window->Flags & ImGuiWindowFlags_Popup))) ? ImVec2(FLT_MAX, FLT_MAX) : ImGui::GetMainViewport()->WorkSize - style.DisplaySafeAreaPadding * 2.0f;
-        const int monitor_idx = window->ViewportAllowPlatformMonitorExtend;
-        if (monitor_idx >= 0 && monitor_idx < g.PlatformIO.Monitors.Size && (window->Flags & ImGuiWindowFlags_ChildWindow) == 0)
-            size_max = g.PlatformIO.Monitors[monitor_idx].WorkSize - style.DisplaySafeAreaPadding * 2.0f;
+        ImVec2 size_max = ImVec2(FLT_MAX, FLT_MAX);
+
+        // Child windows are layed within their parent (unless they are also popups/menus) and thus have no restriction
+        if ((window->Flags & ImGuiWindowFlags_ChildWindow) == 0 || (window->Flags & ImGuiWindowFlags_Popup) != 0)
+        {
+            if (!window->ViewportOwned)
+                size_max = ImGui::GetMainViewport()->WorkSize - style.DisplaySafeAreaPadding * 2.0f;
+            const int monitor_idx = window->ViewportAllowPlatformMonitorExtend;
+            if (monitor_idx >= 0 && monitor_idx < g.PlatformIO.Monitors.Size)
+                size_max = g.PlatformIO.Monitors[monitor_idx].WorkSize - style.DisplaySafeAreaPadding * 2.0f;
+        }
+
         ImVec2 size_auto_fit = ImClamp(size_desired, size_min, ImMax(size_min, size_max));
 
         // FIXME: CalcWindowAutoFitSize() doesn't take into account that only one axis may be auto-fit when calculating scrollbars,