Clarified usage of ListBoxHeader() before we rename those functions + fixed demo code that didn't honor it correctly. (#1783)

imgui.h

@@ -407,9 +407,9 @@ namespace ImGui
     IMGUI_API bool          Selectable(const char* label, bool* p_selected, ImGuiSelectableFlags flags = 0, const ImVec2& size = ImVec2(0,0));       // "bool* p_selected" point to the selection state (read-write), as a convenient helper.
     IMGUI_API bool          ListBox(const char* label, int* current_item, const char* const items[], int items_count, int height_in_items = -1);
     IMGUI_API bool          ListBox(const char* label, int* current_item, bool (*items_getter)(void* data, int idx, const char** out_text), void* data, int items_count, int height_in_items = -1);
-    IMGUI_API bool          ListBoxHeader(const char* label, const ImVec2& size = ImVec2(0,0)); // use if you want to reimplement ListBox() will custom data or interactions. make sure to call ListBoxFooter() afterwards.
+    IMGUI_API bool          ListBoxHeader(const char* label, const ImVec2& size = ImVec2(0,0)); // use if you want to reimplement ListBox() will custom data or interactions. if the function return true, you can output elements then call ListBoxFooter() afterwards.
     IMGUI_API bool          ListBoxHeader(const char* label, int items_count, int height_in_items = -1); // "
-    IMGUI_API void          ListBoxFooter();                                                    // terminate the scrolling region
+    IMGUI_API void          ListBoxFooter();                                                    // terminate the scrolling region. only call ListBoxFooter() if ListBoxHeader() returned true!
 
     // Widgets: Value() Helpers. Output single value in "name: value" format (tip: freely declare more in your code to handle your types. you can add functions to the ImGui namespace)
     IMGUI_API void          Value(const char* prefix, bool b);

imgui_demo.cpp

@@ -1299,10 +1299,12 @@ void ImGui::ShowDemoWindow(bool* p_open)
             ImGui::Button("LEVERAGE\nBUZZWORD", size);
             ImGui::SameLine();
 
-            ImGui::ListBoxHeader("List", size);
-            ImGui::Selectable("Selected", true);
-            ImGui::Selectable("Not Selected", false);
-            ImGui::ListBoxFooter();
+            if (ImGui::ListBoxHeader("List", size))
+            {
+                ImGui::Selectable("Selected", true);
+                ImGui::Selectable("Not Selected", false);
+                ImGui::ListBoxFooter();
+            }
 
             ImGui::TreePop();
         }