Column selection in LibreOffice Calc is very slow with GTK3

[suokunlong]

Column selection in a Calc spreadsheet is very slow under gtk3.

Steps to Reproduce:

1. Open the attached ODS file with LibreOffice Calc in gtk3 mode.

("Help > About LibreOffice" should show "VCL: gtk3". If not, "export SAL_USE_VCLPLUGIN=gtk3" and start libreoffice from terminal)

2. Try to select the entire columns A:G.
--> The selection is very slow.

3. Open the ODS file in "gen" mode, then select columns A:G.
("export SAL_USE_VCLPLUGIN=gen" and start libreoffice from terminal)
--> The selection is fast.

This bug was initially reported on the LibreOffice bugzilla, but it is suspected to be specific to Ubuntu 18.04 and gtk3, so I raise this issue here.
https://bugs.documentfoundation.org/show_bug.cgi?id=129915

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: libreoffice-core 1:6.0.7-0ubuntu0.18.04.10
ProcVersionSignature: Ubuntu 5.0.0-37.40~18.04.1-generic 5.0.21
Uname: Linux 5.0.0-37-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.9
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Sun Jan 12 18:52:45 2020
ExecutablePath: /usr/lib/libreoffice/program/soffice.bin
InstallationDate: Installed on 2019-12-14 (28 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
SourcePackage: libreoffice
UpgradeStatus: No upgrade log present (probably fresh install)

[suokunlong (suokunlong)]

By the way, the slowness is much worse in LibreOffice version 6.2 and above. In the ubuntu build version (which is version 6.0.7), the slowness is not too bad, but still significant as compared to non-gtk3 mode.