Windows: fixed an issue where resizable child windows would emit border logic when hidden/non-visible. (#8815)

This makes more sense in docking branch but may be merged in master right away.

docs/CHANGELOG.txt

@@ -43,6 +43,9 @@ Breaking Changes:
 
 Other Changes:
 
+- Windows: fixed an issue where resizable child windows would emit border
+  logic when hidden/non-visible (e.g. when in a docked window that is not
+  selected), impacting code not checking for BeginChild() return value. (#8815)
 - Error Handling: minor improvements to error handling for TableGetSortSpecs()
   and TableSetBgColor() calls. (#1651, #8499)
 - Backends: OpenGL2, OpenGL3: set GL_UNPACK_ALIGNMENT to 1 before updating 

imgui.cpp

@@ -7578,12 +7578,19 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         }
 #endif
 
+        // Decide if we are going to handle borders and resize grips
+        // 'window->SkipItems' is not updated yet so for child windows we rely on ParentWindow to avoid submitting decorations. (#8815)
+        // Whenever we add support for full decorated child windows we will likely make this logic more general.
+        bool handle_borders_and_resize_grips = true;
+        if ((flags & ImGuiWindowFlags_ChildWindow) && window->ParentWindow->SkipItems)
+            handle_borders_and_resize_grips = false;
+
         // Handle manual resize: Resize Grips, Borders, Gamepad
         int border_hovered = -1, border_held = -1;
         ImU32 resize_grip_col[4] = {};
         const int resize_grip_count = ((flags & ImGuiWindowFlags_ChildWindow) && !(flags & ImGuiWindowFlags_Popup)) ? 0 : g.IO.ConfigWindowsResizeFromEdges ? 2 : 1; // Allow resize from lower-left if we have the mouse cursor feedback for it.
         const float resize_grip_draw_size = IM_TRUNC(ImMax(g.FontSize * 1.10f, window->WindowRounding + 1.0f + g.FontSize * 0.2f));
-        if (!window->Collapsed)
+        if (handle_borders_and_resize_grips && !window->Collapsed)
             if (int auto_fit_mask = UpdateWindowManualResize(window, size_auto_fit, &border_hovered, &border_held, resize_grip_count, &resize_grip_col[0], visibility_rect))
             {
                 if (auto_fit_mask & (1 << ImGuiAxis_X))
@@ -7720,7 +7727,6 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
             // Handle title bar, scrollbar, resize grips and resize borders
             const ImGuiWindow* window_to_highlight = g.NavWindowingTarget ? g.NavWindowingTarget : g.NavWindow;
             const bool title_bar_is_highlight = want_focus || (window_to_highlight && window->RootWindowForTitleBarHighlight == window_to_highlight->RootWindowForTitleBarHighlight);
-            const bool handle_borders_and_resize_grips = true; // This exists to facilitate merge with 'docking' branch.
             RenderWindowDecorations(window, title_bar_rect, title_bar_is_highlight, handle_borders_and_resize_grips, resize_grip_count, resize_grip_col, resize_grip_draw_size);
 
             if (render_decorations_in_parent)