Scrollbar: layout needs to take account of window border size, so a border size will slightly reduce scrollbar size. (#2522)

Rework/revert intent of c1a61d25a.

docs/CHANGELOG.txt

@@ -114,6 +114,9 @@ Other changes:
 - InputText: Fixed a case where deactivation frame would write to underlying
   buffer or call CallbackResize although unnecessary, in a frame where the
   return value was false.
+- Scrollbar: layout needs to take account of window border size, so a border size
+  will slightly reduce scrollbar size. Generally we tried to make it that window
+  border size has no incidence on layout but this can't work with thick borders. (#2522)
 - IO: Added io.ClearEventsQueue() to clear incoming inputs events. (#4921)
   May be useful in conjunction with io.ClearInputsKeys() if you need to clear
   both current inputs state and queued events (e.g. when using blocking native

imgui_widgets.cpp

@@ -881,9 +881,9 @@ ImRect ImGui::GetWindowScrollbarRect(ImGuiWindow* window, ImGuiAxis axis)
     const float scrollbar_size = window->ScrollbarSizes[axis ^ 1]; // (ScrollbarSizes.x = width of Y scrollbar; ScrollbarSizes.y = height of X scrollbar)
     IM_ASSERT(scrollbar_size > 0.0f);
     if (axis == ImGuiAxis_X)
-        return ImRect(inner_rect.Min.x, ImMax(outer_rect.Min.y, outer_rect.Max.y - border_size - scrollbar_size), inner_rect.Max.x, outer_rect.Max.y);
+        return ImRect(inner_rect.Min.x, ImMax(outer_rect.Min.y, outer_rect.Max.y - border_size - scrollbar_size), inner_rect.Max.x - border_size, outer_rect.Max.y - border_size);
     else
-        return ImRect(ImMax(outer_rect.Min.x, outer_rect.Max.x - border_size - scrollbar_size), inner_rect.Min.y, outer_rect.Max.x, inner_rect.Max.y);
+        return ImRect(ImMax(outer_rect.Min.x, outer_rect.Max.x - border_size - scrollbar_size), inner_rect.Min.y, outer_rect.Max.x - border_size, inner_rect.Max.y - border_size);
 }
 
 void ImGui::Scrollbar(ImGuiAxis axis)