Weird modal behavior of modal window bar resizing main window

[Bartłomiej Żogała]

Steps to reproduce:
1. Create new spreedsheet
2. Click maximize button on Libre Office bar
3. Create sample data suitable for creating pie chart
4. select input data for Piechart
5. Click Pie Chart button, dialog box would apper
6. Do some initial setup of pie chart so it will update chart in main window realtime
7. In my case I don't see full chart in main window, trying to move dialog window to adapt the settings.
8. Now, the weird thing - while I try to move modal window bar, main window goes from maximized state to fixed size, the size and position of the windows is exactly where dialog box is placed. The only way to see spreadsheet again is to either accept or cancel changes in modal Window and maximize main window despite the fact it was in such state before

There is only one window from Gnome display manager perspective and I'm aware that changing the chart creator to seperate window is bigger change but I totally don't understand why it touches main window size while not instructed to do so. This is one of those little things where people get frustrated and replacing open source for commercial solutions.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: libreoffice (not installed)
ProcVersionSignature: Ubuntu 4.15.0-115.116-generic 4.15.18
Uname: Linux 4.15.0-115-generic x86_64
NonfreeKernelModules: vboxnetflt vboxnetadp vboxdrv wl
ApportVersion: 2.20.9-0ubuntu7.17
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Thu Sep 10 18:19:59 2020
InstallationDate: Installed on 2015-05-08 (1952 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=pl_PL.UTF-8
 SHELL=/bin/bash
SourcePackage: libreoffice
UpgradeStatus: Upgraded to bionic on 2018-08-26 (745 days ago)