Creator's UI is too small on high DPI displays on Linux

I'm creating this as a bug and not a suggestion because the workarounds for this are ridiculous and we shouldn't subject users to them.

I just installed Ubuntu 18.04 recently and as expected, Creator's UI is too small. QT_AUTO_SCREEN_SCALE_FACTOR=1 works fine for me, and always has, but to use it I need to run Creator from a script. In 18.04, I can no longer press Alt+F2 and run a script, and running Creator from the terminal and putting it in the background (not sure if that term is correct, but I mean: qtcreator &) doesn't work, because when I close the terminal, Creator closes too.

As mentioned here, this means that I can't have Creator on Ubuntu's dock, because even creating a custom .desktop file doesn't work. Users shouldn't have to do all this just to get Creator to be usable.

Apparently Creator did set the variable by default at some stage, but it was reverted. The reasoning being explained here. I haven't experienced any of these issues, and I'm using stock Ubuntu.

I'm proposing that we set this by default, and have an opt-out for it in the form of a checkbox in the settings somewhere for those for whom it doesn't work.

Related stuff below.

https://bugreports.qt.io/browse/QTCREATORBUG-16231?focusedCommentId=319329&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-319329:

It would be great to enable automatic UI Scaling also for Linux. We intentionally skip that for Linux because of QTCREATORBUG-15115 . None of our Linux users here have dare to use a HighDPI screen, so we did not re-evaluate the issue that Thiago with latest Qt versions and Linux versions.
Maybe, it would be a good idea to re-enable it in master (aka 4.1) and test whether people have issues with the binary Qt Creator snapshots.

https://bugreports.qt.io/browse/QTCREATORBUG-16231?focusedCommentId=319529&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-319529

You constitute 50% of all Qt Creator / Linux users worldwide who are known to have a monitor with High DPI (and who use it as intended). And 100% of Cretoror+Linux+HighDPI users who report that it works for them are called Mitch.

Yes, that's how scarce our data regarding that topic is. In contrast we have several Mac and Windows users with HighDPI monitors right in the office. (Linux users are the biggest user group here, though)

Related links:

• https://bugreports.qt.io/browse/QTCREATORBUG-16231
• https://bugreports.qt.io/browse/QTCREATORBUG-17829