Fix T84345: Transforming the cursor fails with absolute grid-snap

Absolute grid snapping was using the pivot, which doesn't make sense
when transforming the cursor.

2 files changed, 19 insertions, 4 deletions

diff --git a/source/blender/editors/transform/transform_convert.c b/source/blender/editors/transform/transform_convert.c
index c81c954bd0a..dd35d67e1fa 100644
--- a/source/blender/editors/transform/transform_convert.c
+++ b/source/blender/editors/transform/transform_convert.c
@@ -993,6 +993,10 @@ void createTransData(bContext *C, TransInfo *t)
     else {
       convert_type = TC_CURSOR_VIEW3D;
     }
+
+    /* Since we're transforming the cursor, initialize this value before it's modified.
+     * Needed for #snap_grid_apply to access the cursor location. */
+    transformCenter_from_type(t, V3D_AROUND_CURSOR);
   }
   else if (!(t->options & CTX_PAINT_CURVE) && (t->spacetype == SPACE_VIEW3D) && ob &&
            (ob->mode == OB_MODE_SCULPT) && ob->sculpt) {
diff --git a/source/blender/editors/transform/transform_snap.c b/source/blender/editors/transform/transform_snap.c
index d407cea0033..e5b16d0ef6e 100644
--- a/source/blender/editors/transform/transform_snap.c
+++ b/source/blender/editors/transform/transform_snap.c
@@ -1424,10 +1424,21 @@ static void snap_grid_apply(
   const float *center_global = t->center_global;
   const float *asp = t->aspect;
 
-  /* use a fallback for cursor selection,
-   * this isn't useful as a global center for absolute grid snapping
-   * since its not based on the position of the selection. */
-  if (t->around == V3D_AROUND_CURSOR) {
+  if (t->options & CTX_CURSOR) {
+    /* Note that we must already have called #transformCenter_from_type, otherwise
+     * we would be lazy-initializing data which is being transformed,
+     * causing the transformed cursor location to be used instead of it's initial location. */
+    BLI_assert(t->center_cache[V3D_AROUND_CURSOR].is_set);
+
+    /* Use a fallback when transforming the cursor.
+     * In this case the center is _not_ derived from the cursor which is being transformed. */
+    const TransCenterData *cd = transformCenter_from_type(t, V3D_AROUND_CURSOR);
+    center_global = cd->global;
+  }
+  else if (t->around == V3D_AROUND_CURSOR) {
+    /* Use a fallback for cursor selection,
+     * this isn't useful as a global center for absolute grid snapping
+     * since its not based on the position of the selection. */
     const TransCenterData *cd = transformCenter_from_type(t, V3D_AROUND_CENTER_MEDIAN);
     center_global = cd->global;
   }