[Qgis-developer] Map canvas flickers

[Matthias Kuhn]

Hi all,

there is this issue report #4011 "Map canvas flickers when content is
dragged" which is actually pretty annoying and doesn't get much
attention. ( http://hub.qgis.org/issues/4011 ) I think the solution
would be pretty simple. Please read on.

It was introduced by f4d26d6211830a866030a333236dcfbf15e077aa "Fix for
resize crash, ticket #2714".
This bugfix disables backbuffering, what fixes the crash but leads to
the beforementioned flickering.

The crash seems to be related to Qt versions (probably 4.6 and 4.7
affected). When I tried to reproduce the crash in 4.8.1 (by reverting
abovementioned patch) I couldn't reproduce the crash. So it is well
possible that this problem has been fixed in Qt upstreams.

But: I myself never could reproduce this crash (due to the lack of a
system with old Qt libs). So, I would be happy if someone familiar with
this problem could confirm that this is fixed with Qt 4.8.

Then there are two possibilities (unless you come up with a third one):

1. Check for qVersion upon every repaint as suggested by Martin Dobias
and then enable/disable the workaround accordingly.

2. Switch the workaround on/off by a pre-compiler constant. This way,
any new distro with Qt>=4.8 could be shipped without the workaround-code
being compiled into the binaries.

Any comments on this issue are very welcome, as it makes QGIS look a lot
sweeter :)

Regards