Windows: clarify/simplify background rendering: alpha should always be zero for DockNodeHost windows. (#2522)

Amend b16f738d04 which AFAIK misleadingly added ImGuiWindowFlags_DockNodeHost tests in there.

imgui.cpp

@@ -7479,15 +7479,20 @@ void ImGui::RenderWindowDecorations(ImGuiWindow* window, const ImRect& title_bar
                     bg_col = (bg_col & ~IM_COL32_A_MASK) | (IM_F32_TO_INT8_SAT(alpha) << IM_COL32_A_SHIFT);
             }
 
-            // Render, for docked windows and host windows we ensure bg goes before decorations
+            // Render, for docked windows and host windows we ensure BG goes before decorations
             if (window->DockIsActive)
                 window->DockNode->LastBgColor = bg_col;
-            ImDrawList* bg_draw_list = window->DockIsActive ? window->DockNode->HostWindow->DrawList : window->DrawList;
-            if (window->DockIsActive || (flags & ImGuiWindowFlags_DockNodeHost))
-                bg_draw_list->ChannelsSetCurrent(DOCKING_HOST_DRAW_CHANNEL_BG);
-            bg_draw_list->AddRectFilled(window->Pos + ImVec2(0, window->TitleBarHeight), window->Pos + window->Size, bg_col, window_rounding, (flags & ImGuiWindowFlags_NoTitleBar) ? 0 : ImDrawFlags_RoundCornersBottom);
-            if (window->DockIsActive || (flags & ImGuiWindowFlags_DockNodeHost))
-                bg_draw_list->ChannelsSetCurrent(DOCKING_HOST_DRAW_CHANNEL_FG);
+            if (flags & ImGuiWindowFlags_DockNodeHost)
+                bg_col = 0;
+            if (bg_col & IM_COL32_A_MASK)
+            {
+                ImDrawList* bg_draw_list = window->DockIsActive ? window->DockNode->HostWindow->DrawList : window->DrawList;
+                if (window->DockIsActive)
+                    bg_draw_list->ChannelsSetCurrent(DOCKING_HOST_DRAW_CHANNEL_BG);
+                bg_draw_list->AddRectFilled(window->Pos + ImVec2(0, window->TitleBarHeight), window->Pos + window->Size, bg_col, window_rounding, (flags & ImGuiWindowFlags_NoTitleBar) ? 0 : ImDrawFlags_RoundCornersBottom);
+                if (window->DockIsActive)
+                    bg_draw_list->ChannelsSetCurrent(DOCKING_HOST_DRAW_CHANNEL_FG);
+            }
         }
         if (window->DockIsActive)
             window->DockNode->IsBgDrawnThisFrame = true;