Disable interpolation of customdata layers on edge slide

This option might be useful in some cases like keeping UV coordinated in kind of
"correct" state so sliding edge loop will also slide edges in UV. But for other
layers like shapekeys and vertex colors it's arguable it's actually needed.

The way it's currently done isn't actually acceptable because it doesn't take
flags like UVCALC_TRANSFORM_CORRECT into account and also makes extra unwanted
things like interpolating shapekeys which leads to propagating changes into
basis keys and also results with slide applied twice for relative keys because
offset is also applying on loading edit mesh.

Also discussed with Brecht who agreed with marking this things as TODO and
disabling for now.

This should fix issue reported in #30387: Edge Slide propogates to Basis Shape Key

2 files changed, 13 insertions, 2 deletions

diff --git a/source/blender/editors/transform/transform.c b/source/blender/editors/transform/transform.c
index dc012619ba4..8c690268444 100644
--- a/source/blender/editors/transform/transform.c
+++ b/source/blender/editors/transform/transform.c
@@ -4721,7 +4721,10 @@ void projectSVData(TransInfo *t, int final)
 						continue;  /* shouldn't happen, but protection */
 				}
 				
-				BM_loop_interp_from_face(em->bm, l2, copyf, do_vdata, FALSE);
+				/* do not run interpolation of all layers for now because it's not actually what you'll always expect
+				 * and layers like shapekeys shouldn't be interpolated from here because oherwise they'll
+				 * propagate to basis keys and will propagate twice to related keys (sergey) */
+				// BM_loop_interp_from_face(em->bm, l2, copyf, do_vdata, FALSE);
 
 				if (final) {
 					BM_loop_interp_multires(em->bm, l2, copyf);	
@@ -4855,7 +4858,10 @@ static int doEdgeSlide(TransInfo *t, float perc)
 		}
 	}
 	
-	projectSVData(t, 0);
+	/* BMESH_TODO: simply not all layers should be interpolated from there
+	 * but it's quite complicated to set this up with current API.
+	 * details are in comments in projectSVData function */
+	// projectSVData(t, 0);
 	
 	return 1;
 }
diff --git a/source/blender/editors/transform/transform_conversions.c b/source/blender/editors/transform/transform_conversions.c
index 93be921a2bd..4e51462921c 100644
--- a/source/blender/editors/transform/transform_conversions.c
+++ b/source/blender/editors/transform/transform_conversions.c
@@ -4889,6 +4889,10 @@ void special_aftertrans_update(bContext *C, TransInfo *t)
 				}
 				EDBM_automerge(t->scene, t->obedit, 1);
 			}
+#if 0
+			/* BMESH_TODO: simply nothing to cancel from here, but when interpolation of
+			 * some custom layers would be added this code would eb needed
+			 * some details are in comments in projectSVData (sergey) */
 			else {
 				if (t->mode == TFM_EDGE_SLIDE) {
 					SlideData *sld = t->customData;
@@ -4897,6 +4901,7 @@ void special_aftertrans_update(bContext *C, TransInfo *t)
 					projectSVData(t, FALSE);
 				}
 			}
+#endif
 		}
 	}