the desktop window is diminished at the left side, after update

[pascal dasset]

lubuntu 18.04 32bits
working well since two years.
after update the dekstop window is reduced at the left side. a grey band appears at the left side.
the tool bar is ok , it occupies the whole screen.
libre office occupies the whole screen after pressing f11

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: lxde (not installed)
ProcVersionSignature: Ubuntu 5.4.0-117.132~18.04.1-generic 5.4.189
Uname: Linux 5.4.0-117-generic i686
ApportVersion: 2.20.9-0ubuntu7.28
Architecture: i386
CurrentDesktop: LXDE
Date: Wed Jun 8 11:24:13 2022
InstallationDate: Installed on 2020-11-16 (568 days ago)
InstallationMedia: Lubuntu 18.04.5 LTS "Bionic Beaver" - Release i386 (20200806.1)
ProcEnviron:
 LANGUAGE=fr_FR
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: lxde-metapackages
UpgradeStatus: No upgrade log present (probably fresh install)

[pascal dasset (pascal48)]

i tried to remove the file .config and restarted.
the desktop window is normal, it occupies the whole screen . this seems to solve the proplem.
but what happened ??