using BROWSER environment variable to set default browser creates several problems

[ԜаӀtеr Ⅼарсһуnѕkі]

LONG STORY SHORT
================
LXQt's mechanism to change the default web browser is broken. They should be using `xdg-settings set default-web-browser` instead of setting `$BROWSER`.

ASSOCIATED BUG REPORTS
======================
See here for example test cases:
 * https://bugs.launchpad.net/ubuntu/+source/chromium-browser/+bug/1823917
 * https://bugs.launchpad.net/ubuntu/+source/lxqt-config/+bug/1795539

The upstream GitHub issue is linked.

Also Lubuntu is tracking this here:
https://phab.lubuntu.me/T53

NOTES
=====
 1. Chromium to change to default browser (https://bugs.launchpad.net/ubuntu/+source/chromium-browser/+bug/1823917)
 2. File associations and scheme handler settings to follow the default browser setting
 3. Browsers with no in-application mechanism to change the default browser to become the default browser (though this requires 1 below to be fixed)

Other problems with the current system:
 1. The list of browsers is not complete
 2. You can search for a browser and enter the executable, but it's never added to the list and disappears when something else is selected
 3. If `$BROWSER` is set in /etc/xdg/xdg-Lubuntu/lxqt/session.conf, the environment variable can't be deleted by the UI

WORKAROUND
==========
 1. Remove `$BROWSER` from the /etc/xdg/xdg-Lubuntu/lxqt/session.conf
 2. Remove `$BROWSER` from the $HOME/.config/lxqt/session.conf
 3. Get rid of the environment variable with `unset BROWSER`
 4. Follow these rules for switching browsers:
    * Do NOT use `$BROWSER`, which can be set as a "Default Application" or an "Environment Variable" in `lxqt-config-session` ("Session Settings")
    * To switch between chromium, firefox, or opera as default browsers, you can use the in-application mechanism
    * For browsers without an in-application mechanism:
      1. Find the desktop file in /usr/share/applications associated with the browser, e.g. chromium-browser.desktop
      2. Run `xdg-settings set default-web-browser browser.desktop`

ProblemType: Bug
DistroRelease: Ubuntu 19.04
Package: lxqt-session 0.14.1-0ubuntu1
ProcVersionSignature: Ubuntu 5.0.0-11.12-generic 5.0.6
Uname: Linux 5.0.0-11-generic x86_64
ApportVersion: 2.20.10-0ubuntu27
Architecture: amd64
CasperVersion: 1.403
CurrentDesktop: LXQt
Date: Sat Apr 13 13:21:58 2019
LiveMediaBuild: Lubuntu 19.04 "Disco Dingo" - Alpha amd64 (20190413.1)
SourcePackage: lxqt-session
UpgradeStatus: No upgrade log present (probably fresh install)

[ԜаӀtеr Ⅼарсһуnѕkі (wxl)]

This doesn't seem to be an issue anymore, at least not in Jammy. The upstream bug is closed with a fix, so that makes sense.