Windows: shallow tweaks in window bg rendering to simplify diff w/ docking.

imgui.cpp

diff --git a/imgui.cpp b/imgui.cpp
index 4813c1b..e8f54ef 100644
--- a/imgui.cpp
+++ b/imgui.cpp
@@ -7129,7 +7129,13 @@
             }
             if (override_alpha)
                 bg_col = (bg_col & ~IM_COL32_A_MASK) | (IM_F32_TO_INT8_SAT(alpha) << IM_COL32_A_SHIFT);
-            window->DrawList->AddRectFilled(window->Pos + ImVec2(0, window->TitleBarHeight), window->Pos + window->Size, bg_col, window_rounding, (flags & ImGuiWindowFlags_NoTitleBar) ? 0 : ImDrawFlags_RoundCornersBottom);
+            if (bg_col & IM_COL32_A_MASK)
+            {
+                ImRect bg_rect(window->Pos + ImVec2(0, window->TitleBarHeight), window->Pos + window->Size);
+                ImDrawFlags bg_rounding_flags = (flags & ImGuiWindowFlags_NoTitleBar) ? 0 : ImDrawFlags_RoundCornersBottom;
+                ImDrawList* bg_draw_list = window->DrawList;
+                bg_draw_list->AddRectFilled(bg_rect.Min, bg_rect.Max, bg_col, window_rounding, bg_rounding_flags);
+            }
         }
 
         // Title bar
@@ -12781,6 +12787,7 @@
 }
 
 // Note technically focus related but rather adjacent and close to BringWindowToFocusFront()
+// FIXME-FOCUS: Could opt-in/opt-out enable modal check like in FocusWindow().
 void ImGui::BringWindowToDisplayFront(ImGuiWindow* window)
 {
     ImGuiContext& g = *GImGui;