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I can't reproduce with your file. It is a multipart polygon. Is that result of the split or is the original file?

It is the result of the "split parts", can you select the feature clicking in the upper "half" rectangle?
i can not...

I noted that the "split parts" tool is not working properly also using multipart geometries:

1. Create a new polygon shapefile,
2. draw a polygon (id 1),
3. draw a second polygon (id 2) totally outside the first one,
4. select both polygons,
5. merge the two polygons with the "merge selected features" tool, now we have a multipart geometry,
6. split one of the two polygons with the split parts tool -> at this point you get an invalid geometry
7. save and toggle edit off

you can verify that you get an invalid geometry (e.g. with the QGIS topology checker)

thanks, Roy.

6 - split one of the two polygons with the split parts tool -> at this point you get an invalid geometry

At this point, the splitted parts have at least one segment in common, so it's effectively an invalid geometry.

The geometries are effectively invalid, this is not a bug, split parts makes invalid geometries, it is a logical consequence.

For the selection problem, I suspect that this is due to OGR.
The selection tool use a QgsFeatureRequest with a filterRect using the spatial index.
The QgsFeatureRequest iterator do not return the invalid feature for one of its parts.
If I'm not mistaken, the spatial index is applied by OGR.

@Giovanni: Can you update the title:
QgsFeatureRequest iterator using filterRect (spatial index) lose some feature with invalid geometries which have one segment in common.

Ticket created on GDAL #5974

Did this change after merging of the new geometry classes? Apparently split parts is not generating invalid geometry anymore - it's now not splitting, but cutting - leaving only one part of the split geometry. So now it's worse.

I understand that it was producing invalid geometries before - which it actually must - polygons are not allowed to have common edges, which the initial cut edges have to be. The geometries must be edited afterwards. Eg. the initial corn polygon could be a result of splitting an earlier single part polygon, that was covering both parts and the space in between, twice and removing the middle part. So for that you even need to have the split parts tool available for single part geometries.

Jürgen Fischer wrote:

Did this change after merging of the new geometry classes? Apparently split parts is not generating invalid geometry anymore - it's now not splitting, but cutting - leaving only one part of the split geometry. So now it's worse.

yes something changed, but i need to do some tests to understand the new behaviour ...

The geometries must be edited afterwards.

you cannot edit invalid geometries in fact, QGIS properly complains that the geometry is invalid.

Eg. the initial corn polygon could be a result of splitting an earlier single part polygon, that was covering both parts and the space in between, twice and removing the middle part.

nope, the initial corn polygon could not be a result of splitting an earlier single part polygon twice,
because at the first go you produce an invalid geometry and you are stuck with that , with QGIS complaining
that the geometry is invalid (as it should indeed)... i hope the issue is more clear now.

Roy Roge wrote:

you cannot edit invalid geometries in fact, QGIS properly complains that the geometry is invalid.

Generally you can edit invalid geometries - maybe not this one. And that's the bug. I doubt that it's even generally true for any split parts, as it would render the split part tool completely useless.

Eg. the initial corn polygon could be a result of splitting an earlier single part polygon, that was covering both parts and the space in between, twice and removing the middle part.

nope, the initial corn polygon could not be a result of splitting an earlier single part polygon twice,
because at the first go you produce an invalid geometry and you are stuck with that , with QGIS complaining
that the geometry is invalid (as it should indeed)... i hope the issue is more clear now.

I was describing how it's intended to work. Splitting a part without further editing doesn't make sense as polygons describe a set of points and on split that set doesn't change. That probably also why it is considered invalid - it's not the simplest way to describe that set.

The split tool is just a shortcut - if it wasn't there, you would have to manually edit the original part to form one part and add the other one by digitizing.