Hi Lisandro and Maximiliano!

On Wed, Apr 04, 2018 at 09:03:34AM -0300, Lisandro Damián Nicanor Pérez Meyer 
> [08:50:25] <maxy> lisandro, mitya57: yet again about PLATFORMTHEMES,
> currently nothing depends on qt5-gtk-platformtheme qt5-style-plugins nor
> qt5ct, I don't know what qt5 uses when QT_PLATFORMTHEME is not being set,
> but it seems reasonable that at least that package gets installed when
> installing a qt app.
> [08:51:22] <lisandro> let me see if I understand that
> [08:52:02] <lisandro> when there is no QT_PLATFORMTHEME set then qt shows a
> default theme, which I *think* it's the windows' one
> [08:52:52] <lisandro> if the user wants a different theme then she must
> install a new theme or qt5ct in order to easily change the ones shipped by
> qt
> [08:53:52] <lisandro> we could recommend qt5ct with libqt5widgets5 I guess
> [08:55:40] <maxy> That would be fine by me, and qt5ct sets QT_PLATFORMTHEME
> in the session if installed. (that would also avoid desktop environments to
> have to set QT_PLATFORMTHEME)
> [08:56:52] <lisandro> so far it sets it everywhere except Plasma
> [08:56:58] <maxy> At least on the cinnamon side of things we are setting
> QT_PLATFORMTHEME=gtk2 as it was the only sensible plaformtheme for stretch,
> but that's no longer true.
> It could be recommends or suggests, thoughts welcomed!

I must admit that qt5ct has significantly improved since the times I last
looked at it. In particular, it now can handle global menus and D-Bus tray

However there is still one major problem with it: it does not support native
GTK+ 3 dialogs. Test case: run Qt Designer under GNOME with qt5ct and without
it. If you click Open button, the native GTK+ dialog will be used only when
qt5ct is not used. This is a significant regression for users of GNOME, Xfce,
Cinnamon, etc (compared to normal Qt setup).

Given this issue, and the fact that it is difficult for an average user to
disable qt5ct, I think we should not recommend it in Qt. Suggests is fine

As Maximiliano mentioned some other things in his IRC message, I will reply
to them too:

> currently nothing depends on qt5-gtk-platformtheme qt5-style-plugins

libqt5gui5 currently recommends qt5-gtk-platformtheme. We cannot depend on
it because we do not want to depend on GTK+.

qt5-style-plugins is a collection of obsolete styles that are no longer
maintained or supported by The Qt Company. It can be sometimes safe to use,
but we do not want to suggest using it.

> At least on the cinnamon side of things we are setting QT_PLATFORMTHEME=gtk2

There is no sense in doing that (and this is the wrong variable name anyway).
Qt automatically enables GTK+ integration support when it is run on Cinnamon
or on other GTK+ based desktop:


Dmitry Shachnev

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