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

[Matthias Kuhn]

Hi,

A fixed date for releasing QGIS with Qt6 sounds like a good idea. And
October 2025 sounds like a good timeline.

We already have a Qt6 version for Windows and the one for macOS should not
be too far off.

I am wondering if we are doing ourselves a favor if we continue to provide
Qt5 builds after the shift.
Wouldn't this prevent us from using pure Qt6 API's and bind development
effort for backwards compatibility?

Alternatively we could extend the lifetime of 3.40 for some more time
(until October 2026 or even February 2027) and only then release the next
LTR.
This would reduce the pressure to migrate immediately and I also wonder if
we want the first ever official Qt6 build to directly become LTR.

Cheers
Matthias

On Tue, Dec 17, 2024 at 1:47 PM Julien Cabieces via QGIS-Developer <
qgis-developer at lists.osgeo.org> wrote:

> > 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".
> >
>
> Agreed, If we were to propose the 2 versions (Qt5 and Qt6) side by side,
> we should propose the Qt 6 version for download sooner than october. Why
> not 3.42 in february?
>
>
> >
> > I will see about making the pkgsrc package have an option for which qt
> > flavor, so I can test too.
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> --
>
> Julien Cabieces
> Senior Developer at Oslandia
> julien.cabieces at oslandia.com
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