fix T49494: snap_align_rotation should use a local pivot to make the transformation

The problem was simple, just transform the global coordinates of t->tsnap.snapTarget to local coordinates.
(Some comments were added to the code)

source/blender/editors/transform/transform.c

@@ -3722,6 +3722,12 @@ static void initRotation(TransInfo *t)
 	copy_v3_v3(t->axis_orig, t->axis);
 }
 
+/**
+ * Applies values of rotation to `td->loc` and `td->ext->quat`
+ * based on a rotation matrix (mat) and a pivot (center).
+ *
+ * Protected axis and other transform settings are taken into account.
+ */
 static void ElementRotation_ex(TransInfo *t, TransData *td, float mat[3][3], const float *center)
 {
 	float vec[3], totmat[3][3], smat[3][3];
@@ -4339,9 +4345,22 @@ static void applyTranslationValue(TransInfo *t, const float vec[3])
 {
 	TransData *td = t->data;
 	float tvec[3];
-	int i;
 
-	for (i = 0; i < t->total; i++, td++) {
+	/* you only need to "apply_snap_align_rotation" when a snap point is found (t->tsnap.status & POINT_INIT)
+	 * so, maybe inside this function is not the best place to apply this rotation.
+	 * but you need "handle snapping rotation before doing the translation" (really?) */
+	const bool apply_snap_align_rotation = usingSnappingNormal(t) && (t->tsnap.status & POINT_INIT);
+	float pivot[3];
+	if (apply_snap_align_rotation) {
+		copy_v3_v3(pivot, t->tsnap.snapTarget);
+		/* The pivot has to be in local-space (see T49494) */
+		if (t->flag & (T_EDIT | T_POSE)) {
+			Object *ob = t->obedit ? t->obedit : t->poseobj;
+			mul_m4_v3(ob->imat, pivot);
+		}
+	}
+
+	for (int i = 0; i < t->total; i++, td++) {
 		if (td->flag & TD_NOACTION)
 			break;
 		
@@ -4352,7 +4371,7 @@ static void applyTranslationValue(TransInfo *t, const float vec[3])
 		bool use_rotate_offset = false;
 
 		/* handle snapping rotation before doing the translation */
-		if (usingSnappingNormal(t)) {
+		if (apply_snap_align_rotation) {
 			float mat[3][3];
 
 			if (validSnappingNormal(t)) {
@@ -4370,7 +4389,7 @@ static void applyTranslationValue(TransInfo *t, const float vec[3])
 				unit_m3(mat);
 			}
 
-			ElementRotation_ex(t, td, mat, t->tsnap.snapTarget);
+			ElementRotation_ex(t, td, mat, pivot);
 
 			if (td->loc) {
 				use_rotate_offset = true;

source/blender/editors/transform/transform.h

@@ -84,8 +84,8 @@ typedef struct TransSnap {
 	bool	peel;
 	bool	snap_spatial_grid;
 	short  	status;
-	float	snapPoint[3]; /* snapping from this point */
+	float	snapPoint[3]; /* snapping from this point (in global-space)*/
-	float	snapTarget[3]; /* to this point */
+	float	snapTarget[3]; /* to this point (in global-space)*/
 	float	snapNormal[3];
 	char	snapNodeBorder;
 	ListBase points;