Scrollbar flicker fix for menus/popups (fixes 4d00dd8326 and 2df8fa95df). Hopefully right this time.

imgui.cpp

@@ -4242,17 +4242,6 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         window->SizeContents.x = (float)(int)((window->SizeContentsExplicit.x != 0.0f) ? window->SizeContentsExplicit.x : ((window_is_new ? 0.0f : window->DC.CursorMaxPos.x - window->Pos.x) + window->Scroll.x));
         window->SizeContents.y = (float)(int)((window->SizeContentsExplicit.y != 0.0f) ? window->SizeContentsExplicit.y : ((window_is_new ? 0.0f : window->DC.CursorMaxPos.y - window->Pos.y) + window->Scroll.y));
 
-        // Update scrollbar status based on the Size that was effective during last frame (and not the upcoming Size which we are updating below), so that user code consuming exactly the available size won't trigger scrollbars when e.g. manually resizing.
-        if (!window->Collapsed)
-        {
-            window->ScrollbarY = (flags & ImGuiWindowFlags_AlwaysVerticalScrollbar) || ((window->SizeContents.y > window->SizeFull.y + style.ItemSpacing.y) && !(flags & ImGuiWindowFlags_NoScrollbar));
-            window->ScrollbarX = (flags & ImGuiWindowFlags_AlwaysHorizontalScrollbar) || ((window->SizeContents.x > window->SizeFull.x - (window->ScrollbarY ? style.ScrollbarSize : 0.0f) - window->WindowPadding.x) && !(flags & ImGuiWindowFlags_NoScrollbar) && (flags & ImGuiWindowFlags_HorizontalScrollbar));
-            if (window->ScrollbarX && !window->ScrollbarY)
-                window->ScrollbarY = (window->SizeContents.y > window->SizeFull.y + style.ItemSpacing.y - style.ScrollbarSize) && !(flags & ImGuiWindowFlags_NoScrollbar);
-            window->ScrollbarSizes = ImVec2(window->ScrollbarY ? style.ScrollbarSize : 0.0f, window->ScrollbarX ? style.ScrollbarSize : 0.0f);
-            window->BorderSize = (flags & ImGuiWindowFlags_ShowBorders) ? 1.0f : 0.0f;
-        }
-
         // Hide popup/tooltip window when first appearing while we measure size (because we recycle them)
         if (window->HiddenFrames > 0)
             window->HiddenFrames--;
@@ -4303,6 +4292,21 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         // Apply minimum/maximum window size constraints and final size
         window->SizeFull = CalcSizeFullWithConstraint(window, window->SizeFull);
         window->Size = window->Collapsed ? window->TitleBarRect().GetSize() : window->SizeFull;
+        if ((flags & ImGuiWindowFlags_ChildWindow) && !(flags & ImGuiWindowFlags_Popup))
+            window->Size = window->SizeFull;
+
+        // SCROLLBAR STATUS
+
+        // Update scrollbar status (based on the Size that was effective during last frame or the auto-resized Size). We need to do this before manual resize (below) is effective.
+        if (!window->Collapsed)
+        {
+            window->ScrollbarY = (flags & ImGuiWindowFlags_AlwaysVerticalScrollbar) || ((window->SizeContents.y > window->SizeFull.y + style.ItemSpacing.y) && !(flags & ImGuiWindowFlags_NoScrollbar));
+            window->ScrollbarX = (flags & ImGuiWindowFlags_AlwaysHorizontalScrollbar) || ((window->SizeContents.x > window->SizeFull.x - (window->ScrollbarY ? style.ScrollbarSize : 0.0f) - window->WindowPadding.x) && !(flags & ImGuiWindowFlags_NoScrollbar) && (flags & ImGuiWindowFlags_HorizontalScrollbar));
+            if (window->ScrollbarX && !window->ScrollbarY)
+                window->ScrollbarY = (window->SizeContents.y > window->SizeFull.y + style.ItemSpacing.y - style.ScrollbarSize) && !(flags & ImGuiWindowFlags_NoScrollbar);
+            window->ScrollbarSizes = ImVec2(window->ScrollbarY ? style.ScrollbarSize : 0.0f, window->ScrollbarX ? style.ScrollbarSize : 0.0f);
+            window->BorderSize = (flags & ImGuiWindowFlags_ShowBorders) ? 1.0f : 0.0f;
+        }
 
         // POSITION
 
@@ -4316,7 +4320,6 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         {
             IM_ASSERT(window_size_set_by_api); // Submitted by BeginChild()
             window->Pos = window->PosFloat = parent_window->DC.CursorPos;
-            window->Size = window->SizeFull;
         }
 
         const bool window_pos_with_pivot = (window->SetWindowPosVal.x != FLT_MAX && window->HiddenFrames == 0);
@@ -4485,6 +4488,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         window->ContentsRegionRect.Max.y = -window->Scroll.y - window->WindowPadding.y + (window->SizeContentsExplicit.y != 0.0f ? window->SizeContentsExplicit.y : (window->Size.y - window->ScrollbarSizes.y)); 
 
         // Setup drawing context
+        // (NB: That term "drawing context / DC" lost its meaning a long time ago. Initially was meant to hold transient data only. Nowadays difference between window-> and window->DC-> is dubious.)
         window->DC.IndentX = 0.0f + window->WindowPadding.x - window->Scroll.x;
         window->DC.GroupOffsetX = 0.0f;
         window->DC.ColumnsOffsetX = 0.0f;