Critical warning when dragging a selection handle

[su_v]

In current lp:inkscape (trunk) and lp:inkscape/0.92.x, every time a selection handle (to resize, rotate or skew a selection) is grabbed and dragged, there is a critical warning on the console:

** (inkscape:52580): CRITICAL **: void Inkscape::SelTrans::grab(const Geom::Point &, gdouble, gdouble, bool, bool): assertion '!_grabbed' failed

Steps to reproduce:
1) launch inkscape trunk or 0.92.x
2) draw a rectangle
3) drag one of the selection handles
   (e.g. to stretch or rotate the rectangle)

Based on tests with archived 0.92.x builds (on OS X 10.7.5):
- not reproduced with rev <= 15237,
- reproduced with rev >= 15238;
these warnings seem to have been exposed with the backport of r15293 from trunk in r15238:
https://bazaar.launchpad.net/~inkscape.dev/inkscape/0.92.x/changes/15238

Revision 15293: Add triangle knot.
https://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/revision/15293

Reproduced with lp:inkscape r15369 on Ubuntu 14.04 and lp:inkscape/0.92.x r15284 on OS X 10.7.5.

[Alvin Penner (apenner)]

confirmed on Windows 10, Inkscape 0.92pre4 15278 (32 bit exe)

[Jabiertxof (jabiertxof)]

Attached a fix proposal

[su_v (suv-lp)]

Patch tested successfully with lp:inkscape/0.92.x r15284 on OS X 10.7.5 (warnings are gone, AFAICT all transformations of a selection with the mouse by dragging a selection handle work as before; dragging mesh triangle knots is ok, using transformation handles in the node tool context is ok too).

[su_v (suv-lp)]

@JazzyNico - could you please milestone to 0.92? It would be great if this small fix could land in the stable release branch before the final 0.92 is cut.

[Jabiertxof (jabiertxof)]

Fixed in 0.92.x r.15288 and in trunk in r15371