Request: Distribution (align and distribute) takes into account stack order

Migrated from: https://bugs.launchpad.net/inkscape/+bug/1587045

Steps to reproduce:

• open Inkscape
• Create an object
• Duplicate so there are 17 identical objects in the same location
• Move the top object down on the canvas a bit (preferably with enough space to distribute all of them and have them separated by a small amount)
• Select all the objects
• Open Align and Distribute dialog
• Distribute centers equidistantly vertically

Svg from original issue (there is flowed test not visible in browser):

What happened?

Selecting the items from top to bottom shows that the items are not distributed by stacking order (but appear to be ordered seemingly randomly)

Note: the original issue mentioned >16 objects causing this error. I replicated with below 16 objects (I think 10? I forget).

What should have happened?

The stacking order should be taken into account when distributing items.

I think this is the solution. Posting this issue anyway because a) let's double check first, and b) for clearer reference.

diff --git a/src/ui/dialog/align-and-distribute.cpp b/src/ui/dialog/align-and-distribute.cpp
index 1355849325..28bc8f6040 100644
--- a/src/ui/dialog/align-and-distribute.cpp
+++ b/src/ui/dialog/align-and-distribute.cpp
@@ -330,7 +330,7 @@ private :
             }
         }
         //sort bbox by anchors
-        std::sort(sorted.begin(), sorted.end());
+        std::stable_sort(sorted.begin(), sorted.end());
 
         // see comment in ActionAlign above
         int saved_compensation = prefs->getInt("/options/clonecompensation/value", SP_CLONE_COMPENSATION_UNMOVED);

Edit: also investigating other places where this behaviour should be used. Also std::sort(..., ..., sp_repr_compare_position_bool); may be more appropriate (haven't tested)

Inkscape Version and Operating System:

• 1.0alpha2 (e8017634da, 2019-07-17) Linux Mint 19.1