undocked sidebars cannot be moved

[PeterPall]

This bug is Ubuntu focal-only and a few weeks ago it didn't happen.

How to reproduce:
- install wx3.0-examples
- go into the aui sample's folder
- compile the sample
- run the sample
- move the tree sidebar out of the main window
- try to move the tree sidebare again.
At least on my computer there is no way to archive the last step. My experience is that with wxWidgets your mileage might vary with the session type. Mine is:

DESKTOP_SESSION=ubuntu-wayland

Why this is bad? Many wxWidgets-based applications (for example wxMaxima) use detachable sidebars - that currently cannot be attached again.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: wx3.0-headers 3.0.4+dfsg-15build1
ProcVersionSignature: Ubuntu 5.4.0-23.27-lowlatency 5.4.29
Uname: Linux 5.4.0-23-lowlatency x86_64
ApportVersion: 2.20.11-0ubuntu26
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Tue Apr 14 13:25:17 2020
Dependencies:

InstallationDate: Installed on 2019-05-26 (324 days ago)
InstallationMedia: Ubuntu 19.04 "Disco Dingo" - Release amd64 (20190416)
PackageArchitecture: all
SourcePackage: wxwidgets3.0
UpgradeStatus: Upgraded to Focal Fossa