Begin: Fixed auto-fit calculation code that predict the presence of ascrollbar so it works in the case size constraints are used. I actually don't need this fix now, but earlier experiment with BeginCombo() required this fix.

imgui.cpp

@@ -3925,12 +3925,13 @@ static ImVec2 CalcSizeAutoFit(ImGuiWindow* window)
     }
     else
     {
-        // Handling case of auto fit window not fitting in screen on one axis, we are growing auto fit size on the other axis to compensate for expected scrollbar. FIXME: Might turn bigger than DisplaySize-WindowPadding.
+        // Handling case of auto fit window not fitting on the screen (on either axis): we are growing the size on the other axis to compensate for expected scrollbar. FIXME: Might turn bigger than DisplaySize-WindowPadding.
         size_auto_fit = ImClamp(window->SizeContents + window->WindowPadding, style.WindowMinSize, ImMax(style.WindowMinSize, g.IO.DisplaySize - g.Style.DisplaySafeAreaPadding));
-        if (size_auto_fit.x < window->SizeContents.x && !(flags & ImGuiWindowFlags_NoScrollbar) && (flags & ImGuiWindowFlags_HorizontalScrollbar))
+        ImVec2 size_auto_fit_after_constraint = CalcSizeFullWithConstraint(window, size_auto_fit);
+        if (size_auto_fit_after_constraint.x < window->SizeContents.x && !(flags & ImGuiWindowFlags_NoScrollbar) && (flags & ImGuiWindowFlags_HorizontalScrollbar))
             size_auto_fit.y += style.ScrollbarSize;
-        if (size_auto_fit.y < window->SizeContents.y && !(flags & ImGuiWindowFlags_NoScrollbar))
-            size_auto_fit.x += style.ScrollbarSize;
+        if (size_auto_fit_after_constraint.y < window->SizeContents.y && !(flags & ImGuiWindowFlags_NoScrollbar))
+            size_auto_fit.x += style.ScrollbarSize * 2.0f;
         size_auto_fit.y = ImMax(size_auto_fit.y - style.ItemSpacing.y, 0.0f);
     }
     return size_auto_fit;