[doc] Add a shape on rectilinear custom edges

Signed-off-by: Florian ROUËNÉ <[email protected]>

CHANGELOG.adoc

@@ -4,7 +4,8 @@
 
 === Shapes
 
--  Add support for edges connected to edges
+- Add support for edges connected to edges
+- Preserve custom edges rectilinearity
 
 
 === Architectural decision records

doc/iterations/2025.4/force_custom_edges_to_stay_rectilinear.adoc

@@ -0,0 +1,45 @@
+= Force Custom Edges to Stay Rectilinear
+
+== Problem
+
+It is possible to add new bendpoints on an edge, but currently moving (or adding) a bendpoint causes the path to lose its rectilinearity.
+
+== Key Result
+
+Regardless of the actions performed on an edge, the resulting path must remain rectilinear.
+
+== Solution
+
+On edge, bendpoint (black-filled circle) are positioned to each segment corner and intermediate point (grey circle) on middle to each segment.
+
+image:images/custom-edges-point-types.png[Custom edges point types, 70%]
+
+Modify the algorithm for moving a bendpoint so that two new bendpoints are created (for one axis) at the positions of the adjacent intermediate point of the moved bendpoint.
+
+image:images/custom-edges-new-bendpoints-one-axis.png[New benbpoints on one axis, 70%]
+
+The same approach could be applied on the two axis in the same drag.
+
+image:images/custom-edges-new-bendpoints-two-axis.png[New bendpoints on two axis, 70%]
+
+Change the behavior of intermediate points so that they do not create new bendpoints but instead move existing adjacent bendpoints along an axis (x or y).
+This axis is perpendicular to the segment in question.
+
+image:images/custom-edges-intermediate-point-translation.png[Intermediate point translation, 60%]
+
+To delete a bendpoint, align two segments to form a single segment (a margin will be implemented to facilitate this action).
+
+image:images/custom-edges-before-bendpoint-suppression.png[Before bendpoint suppression, 50%]
+
+image:images/custom-edges-after-bendpoint-suppression.png[After bendpoint suppression, 50%]
+
+== Cutting Backs
+
+It would be interesting to define the routing of an edge (rectilinear, oblique, etc...).
+
+== Rabbit Holes
+
+* Node handles are currently fixed, so it's not possible to modify the first and last edge segments directly.
+* When the node is moved, the first bendpoint must be updated to preserve rectilinearity of the edge path.
+
+== No-Gos