MenuBar: Fixed an issue where layouting an item in the menu-bar would erroneously egister contents size. (#6789)

In dire need of removing BeginGroup()/EndGroup() from menu-bar code, fo r sanity.

docs/CHANGELOG.txt

@@ -47,6 +47,9 @@ Other changes:
 - InputTextMultiline: Fixed a crash pressing Down on last empty line of a multiline buffer.
   (regression from 1.89.2, only happened in some states). (#6783, #6000)
 - BeginListBox(): Fixed not consuming SetNextWindowXXX data when returning false.
+- MenuBar: Fixed an issue where layouting an item in the menu-bar would erroneously
+  register contents size in a way that would affect the scrolling layer.
+  Was most often noticable when using an horizontal scrollbar. (#6789)
 - Backends: GLFW: Clear emscripten's MouseWheel callback before shutdown. (#6790, #6096, #4019) [@halx99]
 
 

imgui_widgets.cpp

@@ -7035,12 +7035,18 @@ void ImGui::EndMenuBar()
     PopClipRect();
     PopID();
     window->DC.MenuBarOffset.x = window->DC.CursorPos.x - window->Pos.x; // Save horizontal position so next append can reuse it. This is kinda equivalent to a per-layer CursorPos.
-    g.GroupStack.back().EmitItem = false;
-    EndGroup(); // Restore position on layer 0
+
+    // FIXME: Extremely confusing, cleanup by (a) working on WorkRect stack system (b) not using a Group confusingly here.
+    ImGuiGroupData& group_data = g.GroupStack.back();
+    group_data.EmitItem = false;
+    ImVec2 restore_cursor_max_pos = group_data.BackupCursorMaxPos;
+    window->DC.IdealMaxPos.x = ImMax(window->DC.IdealMaxPos.x, window->DC.CursorMaxPos.x - window->Scroll.x); // Convert ideal extents for scrolling layer equivalent.
+    EndGroup(); // Restore position on layer 0 // FIXME: Misleading to use a group for that backup/restore
     window->DC.LayoutType = ImGuiLayoutType_Vertical;
     window->DC.IsSameLine = false;
     window->DC.NavLayerCurrent = ImGuiNavLayer_Main;
     window->DC.MenuBarAppending = false;
+    window->DC.CursorMaxPos = restore_cursor_max_pos;
 }
 
 // Important: calling order matters!