In PGTK, child frames may be obscured by scroll-bars. This happens when
the scrollbar is added after the child frame is created (e.g. by
creating new windows, etc.). The bug may in particular affect people
using child-frame-based completion packages, such as corfu, company-
box, or company-posframe.

From what I could see in the code, in contrast to X, PGTK adds child
frames to the parent frame's edit_widget which is a GtkFixed (see "x-
create-frame"). A GtkFixed doesn't have any defined z-order behavior.
However, widgets seem to get drawn in the order of the internal child
list. Unfortunately, there is no direct way to insert widgets at the
front of this list or otherwise manipulate the list apart from pushing
children to the end or removing them. So a (dirty) workaround would be
to remove all children, add the scrollbar and then add all the removed
widgets again, see the patch.
A better solution may involve something like a GtKOverlay but I don't
know if that's practical in the current implementation.

To replicate the bug in 'emacs -Q':
(progn
  (make-frame
   `((title . "childframe")
     (parent-frame . ,(selected-frame))
     (border-width . 3)
     (internal-border-width . 3)
     (child-frame-border-width . 3)
     (vertical-scroll-bars . nil)
     (horizontal-scroll-bars . nil)
     (menu-bar-lines . 0)
     (tool-bar-lines . 0)
     (tab-bar-lines . 0)
     (minibuffer . nil)
     (undecorated . t)
     (left . 0.5)
     (top . 0.5)
     (width . 20)
     (height . 20)
     (left-fringe . 0)
     (right-fringe . 0)
     (background-color . "blue")))
(split-window-right)
(split-window-right))