[QGIS-Developer] Alternative layer drawing order not honoured in QGIS 3

[Andreas Neumann]

Hi, 

I know that probably not many people use the feature of the "alternative
drawing order" in QGIS - it is kind of a hidden - not so well-known
feature. Normally, QGIS is using the layer rendering order defined in
the "Layers" panel. But one can deliberately reorganize the layer
rendering order in the separate "Layer Order" panel. This is necessary
for more complex cartographic output (e.g. cadastral maps). 

I noticed that an existing 2.18 qgs project opened in 3.0 does two bad
things: 

1. the checkbox "Control rendering order" in the "Layer Order" panel is
set to "off" in QGIS 3, whereas it was on in 2.18 

2. the ordering in the Layer Order panel is completeley mixed up (not as
it was in 2.18). 

I think that this a very important issue to fix for any serious
cartographic output involving many layers. 

My questions: 

1. Can others confirm that issue, that the layer drawing order is messed
up when opening 2.18 files that had alternative layer orders defined? 

2. Who has time and expertise in this code area to fix this issue? 

I can provide a Postgis-Dump and a sample project that demonstrates this
issue. 

See also the screenshot at
https://issues.qgis.org/attachments/download/12554/layer_rendering_order_garbled.png
showing the issue - on the left is QGIS 2.18 on the right there is QGIS
3. 

Issue report at https://issues.qgis.org/issues/18620  

Thanks for your replies,
Andreas
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