gh-154357: Fix flaky wait_until_mapped(full_size=True)

After a test which left the root window without content, the window
manager can resize it to its minimum size.  Tk then remembers that size
as if it were set by "wm geometry" and no longer resizes the toplevel to
fit the content of the following test.

Author: serhiy-storchaka

Lib/test/test_tkinter/support.py

@@ -122,6 +122,7 @@ def wait_until_mapped(widget, timeout=None, *, full_size=False):
         timeout = support.LOOPBACK_TIMEOUT
     deadline = time.monotonic() + timeout
     widget.update_idletasks()
+    reset = False
     while True:
         widget.update()  # drain pending Map/Configure events
         if widget.winfo_ismapped():
@@ -133,6 +134,11 @@ def wait_until_mapped(widget, timeout=None, *, full_size=False):
                 h_ok = widget.winfo_height() > 1
             if w_ok and h_ok:
                 return True
+            if full_size and not reset:
+                # Tk no longer resizes the toplevel to fit its content if
+                # the window manager has resized it.  Undo this.
+                widget.winfo_toplevel().wm_geometry('')
+                reset = True
         if time.monotonic() >= deadline:
             return False
         time.sleep(0.01)