gh-68453: Fix IDLE -s together with -c, -r or - (#157592)

The startup file runs in the subprocess asynchronously, so the command
or script was queued while it was still executing: an "Already
executing" dialog was shown, sys.argv was not set, and the subprocess
was restarted, discarding the startup file. Wait for the startup file
to finish first.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/idlelib/pyshell.py

@@ -1675,6 +1675,10 @@ def main():
         if filename and os.path.isfile(filename):
             shell.interp.execfile(filename)
     if cmd or script:
+        # Let the startup file finish first: the command or script
+        # is to run after it, in its namespace (gh-68453).
+        while shell.executing:
+            root.update()
         shell.interp.runcommand("""if 1:
             import sys as _sys
             _sys.argv = {!r}

Misc/NEWS.d/next/IDLE/2026-09-15-21-20-04.gh-issue-68453.hlxbog.rst

@@ -0,0 +1,3 @@
+Fix running IDLE with the -s option together with -c, -r or -: the command
+or script now runs after the startup file, without an error dialog and
+without restarting the shell.