Fixed scrollbar enable/disable calculation when size is modified programmatically. This is the generalization of the fix in 2df8fa95df.

imgui.cpp

@@ -4064,12 +4064,8 @@ static ImGuiWindow* CreateNewWindow(const char* name, ImVec2 size, ImGuiWindowFl
     window->Flags = flags;
     g.WindowsById.SetVoidPtr(window->ID, window);
 
-    if (flags & ImGuiWindowFlags_NoSavedSettings)
-    {
     // User can disable loading and saving of settings. Tooltip and child windows also don't store settings.
-        window->Size = window->SizeFull = size;
+    if (!(flags & ImGuiWindowFlags_NoSavedSettings))
-    }
-    else
     {
         // Retrieve settings from .ini file
         // Use SetWindowPos() or SetNextWindowPos() with the appropriate condition flag to change the initial position of a window.
@@ -4085,8 +4081,8 @@ static ImGuiWindow* CreateNewWindow(const char* name, ImVec2 size, ImGuiWindowFl
             if (ImLengthSqr(settings->Size) > 0.00001f)
                 size = settings->Size;
         }
-        window->Size = window->SizeFull = size;
     }
+    window->Size = window->SizeFull = window->SizeFullAtLastBegin = size;
 
     if ((flags & ImGuiWindowFlags_AlwaysAutoResize) != 0)
     {
@@ -4468,13 +4464,13 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
 
         // 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.
+        // Update scrollbar status (based on the Size that was effective during last frame or the auto-resized Size). 
         if (!window->Collapsed)
         {
-            window->ScrollbarY = (flags & ImGuiWindowFlags_AlwaysVerticalScrollbar) || ((window->SizeContents.y > window->SizeFull.y) && !(flags & ImGuiWindowFlags_NoScrollbar));
+            window->ScrollbarY = (flags & ImGuiWindowFlags_AlwaysVerticalScrollbar) || ((window->SizeContents.y > window->SizeFullAtLastBegin.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));
+            window->ScrollbarX = (flags & ImGuiWindowFlags_AlwaysHorizontalScrollbar) || ((window->SizeContents.x > window->SizeFullAtLastBegin.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.ScrollbarSize) && !(flags & ImGuiWindowFlags_NoScrollbar);
+                window->ScrollbarY = (window->SizeContents.y > window->SizeFullAtLastBegin.y + style.ScrollbarSize) && !(flags & ImGuiWindowFlags_NoScrollbar);
             window->ScrollbarSizes = ImVec2(window->ScrollbarY ? style.ScrollbarSize : 0.0f, window->ScrollbarX ? style.ScrollbarSize : 0.0f);
         }
 
@@ -4710,6 +4706,9 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
                 window->DrawList->AddLine(title_bar_rect.GetBL()+ImVec2(1,-1), title_bar_rect.GetBR()+ImVec2(-1,-1), GetColorU32(ImGuiCol_Border), style.FrameBorderSize);
         }
 
+        // Store a backup of SizeFull which we will use next frame to decide if we need scrollbars. 
+        window->SizeFullAtLastBegin = window->SizeFull;
+
         // Update ContentsRegionMax. All the variable it depends on are set above in this function.
         window->ContentsRegionRect.Min.x = -window->Scroll.x + window->WindowPadding.x;
         window->ContentsRegionRect.Min.y = -window->Scroll.y + window->WindowPadding.y + window->TitleBarHeight() + window->MenuBarHeight();

imgui_internal.h

@@ -701,6 +701,7 @@ struct IMGUI_API ImGuiWindow
     ImVec2                  Pos;                                // Position rounded-up to nearest pixel
     ImVec2                  Size;                               // Current size (==SizeFull or collapsed title bar size)
     ImVec2                  SizeFull;                           // Size when non collapsed
+    ImVec2                  SizeFullAtLastBegin;                // Copy of SizeFull at the end of Begin. This is the reference value we'll use on the next frame to decide if we need scrollbars.
     ImVec2                  SizeContents;                       // Size of contents (== extents reach of the drawing cursor) from previous frame
     ImVec2                  SizeContentsExplicit;               // Size of contents explicitly set by the user via SetNextWindowContentSize()
     ImRect                  ContentsRegionRect;                 // Maximum visible content position in window coordinates. ~~ (SizeContentsExplicit ? SizeContentsExplicit : Size - ScrollbarSizes) - CursorStartPos, per axis