[QGIS-Developer] QGIS 3.46 builds to use Qt6?

[Greg Troxel]

Nyall Dawson <nyall.dawson at gmail.com> writes:

> On Tue, 17 Dec 2024 at 10:48, Greg Troxel via QGIS-Developer <
> qgis-developer at lists.osgeo.org> wrote:
>
>> I wonder about "less stable" qt5 and "more stable" qt6.  Do we really
>> believe that qgis built on qt6, with no plugins will have fewer crashes
>> and quirks, than the qt5 build?
>
> Yes, I do. Because Qt 5 is not improving any more, but Qt 6 is. An example
> would be when running under Wayland environments on linux -- it's a very
> broken mess on Qt 5 and will never be fixed. On Qt 6 it's only a
> slightly-broken mess, and will likely be non-broken within the next 12
> months. There's a similar situation for apple processors, which never had
> full official support on Qt 5 but ARE fully supported on Qt 6. This gap is
> only getting wider as newer operating system updates and corresponding
> changes break things underneath Qt 5.

> There's also a limited stream of bug fixes getting ported back to Qt 5.15,
> vs those flowing into the supported Qt 6 releases.

That's a good argument.  I have not really dealt with qt6 yet, and my
impression has been that qt5 is quite stable on my platform (which never
had any official support).  But upstream's maintenenance policies are
troubling.

>>  That is surprising to me at this point.
>> Do we still believe that if one assumes "qgis with N random plugins that
>> claim to support qt6"?
>
> (Well, QGIS + **ANY** plugin = a less stable QGIS. 🤣 But that's a
> completely different point)

of course.  I just meant that stability(qgis-only) could have a
different answer than stability(qgis+p1+p7), where p7 not working with
qt6 leads to a 0 score on the stability metric.

>> I expect a qt6 build is kind of like a .0 release, and we would want to
>> have qt6 builds widel avaialable and time for feedback before saying
>> it's stable.
>
> I'd say we're well past the ".0" stage of Qt 6 support. Almost all the core
> functionality is quite well tested at this stage, and third party clients
> (like Mergin and QField) switched to Qt 6 based QGIS builds earlier this
> year. I'm confident that by the time we hit a (potential) October 2025
> milestone that we'll have a very stable Qt 6 build available.

My experience (not in qgis) is that broad releases often uncover
problems not hit by testing with less scope.  Maybe there won't be much
or perhaps even any, but until the broader qgis world, on varying
systems.

Therefore I think it would be good to start offering qt6 builds on the
website before then, at least by July, to let a larger pool of users try
them, marked "for testing" and then the labels can flip to "for use with
old plugins"/"recommended".


I will see about making the pkgsrc package have an option for which qt
flavor, so I can test too.