Viewport: Fixed collapsed windows setting ImGuiViewportFlags_NoRendererClear without making title bar color opaque.

(thanks christopher knorr!)

docs/CHANGELOG.txt

@@ -106,6 +106,13 @@ Other changes:
   from accessing keys. (#5888, #4921, #456)
 - Inputs: fixed moving a window or drag and dropping from capturing mods. (#5888, #4921, #456)
 
+Docking+Viewports Branch:
+
+- Viewport: Fixed collapsed windows setting ImGuiViewportFlags_NoRendererClear without
+  making title bar color opaque, leading to potential texture/fb garbage being visible.
+  Right now as we don't fully support transparent viewports (#2766), so we turn that
+  'TitleBgCollapsed' color opaque just lke we do for 'WindowBG' on uncollapsed windows.
+
 
 -----------------------------------------------------------------------
  VERSION 1.89 (Released 2022-11-15)

imgui.cpp

@@ -6388,9 +6388,11 @@ void ImGui::RenderWindowDecorations(ImGuiWindow* window, const ImRect& title_bar
     if (window->Collapsed)
     {
         // Title bar only
-        float backup_border_size = style.FrameBorderSize;
+        const float backup_border_size = style.FrameBorderSize;
         g.Style.FrameBorderSize = window->WindowBorderSize;
         ImU32 title_bar_col = GetColorU32((title_bar_is_highlight && !g.NavDisableHighlight) ? ImGuiCol_TitleBgActive : ImGuiCol_TitleBgCollapsed);
+        if (window->ViewportOwned)
+            title_bar_col |= IM_COL32_A_MASK; // No alpha (we don't support is_docking_transparent_payload here because simpler and less meaningful, but could with a bit of code shuffle/reuse)
         RenderFrame(title_bar_rect.Min, title_bar_rect.Max, title_bar_col, true, window_rounding);
         g.Style.FrameBorderSize = backup_border_size;
     }
@@ -6407,8 +6409,7 @@ void ImGui::RenderWindowDecorations(ImGuiWindow* window, const ImRect& title_bar
             ImU32 bg_col = GetColorU32(GetWindowBgColorIdx(window));
             if (window->ViewportOwned)
             {
-                // No alpha
-                bg_col = (bg_col | IM_COL32_A_MASK);
+                bg_col |= IM_COL32_A_MASK; // No alpha
                 if (is_docking_transparent_payload)
                     window->Viewport->Alpha *= DOCKING_TRANSPARENT_PAYLOAD_ALPHA;
             }