Simple Scan UI freezes while scanning from second page on

[Mauro]

In Kubuntu 20.04 Simple Scan application shows the following behaviour on my system.
While scanning the first page, all works fine: the UI is responsive and the live preview of the page being scanned is shown.
Starting from the second page on, while the page scanning is in progress the UI is completely frozen and hence no live preview is shown. Trying to click on the application window shows the UI is completely unresponsive. This lasts until around 10 seconds after the page scanning has been completed (and the scanner light has returned to the "home" position).
Apart from this, the application works, but this slows down multi-page scanning process and hurts user experience.

My scanner is a Canon LiDE 110, a flatbed one.

Simple Scan was working perfectly fine in Ubuntu 16.04 and Ubuntu 14.04 on the same system (after upgrading SANE drivers, but for a completely different reason) and was not showing this behaviour.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: simple-scan 3.36.3-0ubuntu0.20.04.0
ProcVersionSignature: Ubuntu 5.4.0-65.73-generic 5.4.78
Uname: Linux 5.4.0-65-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: KDE
Date: Wed Feb 3 13:04:50 2021
InstallationDate: Installed on 2020-10-05 (120 days ago)
InstallationMedia: Kubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
MachineType: Hewlett-Packard HP ProBook 450 G1
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-5.4.0-65-generic root=/dev/mapper/vgkubuntu-root ro quiet splash zswap.enabled=1 zswap.compressor=lz4 vt.handoff=7
SimpleScanLog: [+0,00s] DEBUG: simple-scan.vala:1720: Starting /usr/bin/simple-scan 3.36.3, PID=56452
SourcePackage: simple-scan
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 02/18/2020
dmi.bios.vendor: Hewlett-Packard
dmi.bios.version: L74 Ver. 01.50
dmi.board.name: 1942
dmi.board.vendor: Hewlett-Packard
dmi.board.version: KBC Version 12.F4
dmi.chassis.type: 10
dmi.chassis.vendor: Hewlett-Packard
dmi.modalias: dmi:bvnHewlett-Packard:bvrL74Ver.01.50:bd02/18/2020:svnHewlett-Packard:pnHPProBook450G1:pvrA3009DD10303:rvnHewlett-Packard:rn1942:rvrKBCVersion12.F4:cvnHewlett-Packard:ct10:cvr:
dmi.product.family: 103C_5336AN G=N L=BUS B=HP S=PRO
dmi.product.name: HP ProBook 450 G1
dmi.product.sku: E9Y21EA#ABZ
dmi.product.version: A3009DD10303
dmi.sys.vendor: Hewlett-Packard

[Bartosz Kosiorek (gang65)]

Please run simple scan in debug mode with command:

 simple-scan -d

reproduce the issue, and attach file with output.

[Mauro (mauromol)]

Here it is the output produced by simple-scan -d
I did this:
- start the program
- scan first page (all working fine)
- scan second page (UI frozen)
- wait for the UI to become responsive again
- exit program