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Yes, I cannot edit the geometry with the node tool, works fine in LTR on the same machine.
I get the message "could not snap to a segment on the current layer." when trying to click over the geometry.

You have to click on a node to start the edit though - I don't think clicking within a polygon to start the edit has ever been supported?

Click inside polygon to start is #3752. The snapping message is misleading though - that's a bug.

I am clicking on nodes and I see that message (note it popups without title: Node Tool).

I can confirm also on Windows.

@Nyall: so works as LTR for you in current master? can you select the attached feature with the Select Features tool?
In current master I cannot neither select the feature nor changing or deleting node.

One inner ring has just 3 vertices (the first and the last being the same), and so OGR fails to organize that polygon.

Polygon disappears when zoomed in because non-trivial geometry intersection has to be done in OGR (using GEOS), which fails because GEOS will not construct such invalid geometry.

Select map tool again does not work because GEOS is used to determine selection (interects / contains / distance).

Node tools sometimes works for me, but sometimes it does not snap. First I need to zoom out a bit in order to "initialize" node tool, then it works. To my surprise, snapping in node tool has been rewritten to use the old QgsMapCanvasSnapper class instead of QgsSnappingUtils, which may explain the different behavior.

same problem here with the latest master branch. can you edit the invalid geometry?

I've just tried editing an simple invalid polygon (bow-tie) and it worked in both 2.8.9 and 2.14.3.
Here's the WKT (loaded via QuickWKT): POLYGON)

Will try your shapefile next.

With the attached shapefile I still DO CAN edit vertices with the node tool while I CAN NOT select the feature with the "select tool". This behavior is exactly the same for me in 2.8.9, 2.14.3 and current master (2.15.0-Master 87a560b).

Oh but I finally found a difference: moving a selected vertex behaves differently.
In 2.8.9 you can see the expected vertex move correctly; in 2.14.3 and 2.15.0-Master 87a560b you don't see the result of the vertex move as you move it, but it works fine on saving the layer. Do you confirm this, Salvatore ?

I made another test with a simplified version of the invalid polygon: a polygon with unclosed ring:

010300000001000000030000000000000000000000000000000000000000000000000024400000000000000000000000000000244000000000000024400000000000000000

Pass the above to QuickWKT plugin (for example), then try to edit.
I get the same (bad? or just unexpected ?) experience in 2.8.9, 2.14.3 and 2.16 when moving the lower-left vertex
(the polygon collapses to a line).
I get a different experience when moving the lower-right vertex
(2.8 keeps showing a triangle, 2.14 and 2.15 show an unfilled contour)

Actually, 2.14 behaves the same as 2.8 with the simplified polygon, so the difference is only in master (2.15.0dev)

In no case I got the "could not snap to a segment on the current layer", while I can never select the polygon by clicking in it (as the ring is not closed so GEOS object cannot be constructed).

I guess we could fix this by ensuring rings are closed, but I'm not sure we want this to happen silently to the user.
Wasn't there a "make valid" plugin ?

Code to force-close ring on converting geometries to GEOS is ready:
https://github.com/qgis/QGIS/pull/3212

Fix the difference in node tool behavior AND selectability of the simplified invalid polygon (triangle polygon with 3 vertices only). The Shapefile geometry is still not selectable with that.

Interesting enough, 2.14.3 receives a 4-vertices exterior ring when QgsGeos object is constructed, so it must be "closed" on constructing the QgsCurvePolygonV2 object. Master (2.15.0) instead still sees only 3 vertices at that point.

Here's where 2.14 closes the polygon ring:

#0  QgsLineStringV2::close (this=0x7fff2800ce40) at /usr/src/qgis/qgis-2.14/src/core/geometry/qgslinestringv2.cpp:884
#1  0x00007ffff4828379 in QgsPolygonV2::setExteriorRing (this=0x7fff2800cca0, ring=0x7fff2800ce40)
    at /usr/src/qgis/qgis-2.14/src/core/geometry/qgspolygonv2.cpp:224
#2  0x00007ffff47be18b in QgsCurvePolygonV2::toPolygon (this=0x35148b0) at /usr/src/qgis/qgis-2.14/src/core/geometry/qgscurvepolygonv2.cpp:429
#3  0x00007ffff4800236 in QgsGeos::asGeos (geom=0x35148b0, precision=0) at /usr/src/qgis/qgis-2.14/src/core/geometry/qgsgeos.cpp:1075
#4  0x00007ffff47f7fd9 in QgsGeos::cacheGeos (this=0x7fff452494d0) at /usr/src/qgis/qgis-2.14/src/core/geometry/qgsgeos.cpp:175
#5  0x00007ffff47f7c51 in QgsGeos::QgsGeos (this=0x7fff452494d0, geometry=0x35148b0, precision=0)
    at /usr/src/qgis/qgis-2.14/src/core/geometry/qgsgeos.cpp:138

@strk thank you for all work on this issue.

the original bug was between 2.8.4 and 2.13 (master version). Today I have done a test between 2.8.4 and master (c81b14d) and the main issue is confirmed. With 2.8.4 I can move the vertices of the invalid geometry (it means that I can fix the invalidity manually) while I can't with master version (with the msg "could not snap to a segment on the current layer."). Substantially the node tool works in a way different between LTR and master version.

Tonight I try to compile the 2.8.9 LTR version and I will do a try with that.

Hi Sandro,

I have the same configuration like you (using your project I get the same behavior).

I done more tests on different machines (actually my friends have done):

• Windows 7, QGIS dev (5bb2c7d), GEOS 3.5.0-CAPI-1.9.0:
  • cannot select, cannot edit with node tool and geometry disappears when zooming

• Windows 7, QGIS 2.14.3, GEOS 3.5.0-CAPI-1.9.0:
  • cannot select, cannot edit with node tool and geometry disappears when zooming

• Windows 7, QGIS 2.8.9, GEOS 3.5.0-CAPI-1.9.0:
  • cannot select, can edit with node tool and geometry disappears when zooming

• Debian 8, QGIS 2.14.3, GEOS 3.4.2-CAPI-1.8.2 r3921:
  • cannot select, can edit with node tool and geometry does not disappear when zooming

How about Debian 8 with QGIS-2.8.9 and dev/2.15/5bb2c7d ?
The test you (or your friend) made only show a regression under Windows7 between 2.8.9 and 2.14.3 ...

Do you think you can make an automated test reproducing the regression issue you're seeing ?
As I'm not sure what "can edit" and "cannot edit" mean, exactly (I can move the vertices with the node tool but not see the fille polygon nor the lines connecting those vertices as soon as I move a vertex, for example)