From 64e79035d5133cf15de41aab0ec9a43bcd4ad08a Mon Sep 17 00:00:00 2001
From: omar <omarcornut@gmail.com>
Date: Tue, 14 Nov 2017 23:17:28 +0100
Subject: [PATCH] Scrollbar flicker fix for menus/popups (fixes
 4d00dd8326b288aa0ff87ad6de4975d0578cfd56 and
 2df8fa95dfe3463ece78c56b02dc815c3e71331a). Hopefully right this time.

---
 imgui.cpp | 30 +++++++++++++++++-------------
 1 file changed, 17 insertions(+), 13 deletions(-)

diff --git a/imgui.cpp b/imgui.cpp
index b828d0e9..f0808585 100644
--- a/imgui.cpp
+++ b/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--;
@@ -4303,7 +4292,22 @@ 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
 
         // Position child window
@@ -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;