Fix particle-edit crash on undo/redo

Undo/redo was crashing & asserting.

There ware two bugs:

- Entering particle system edit-mode requires an evaluated depsgraph.

- The active object could get out of sync when undo/redo
  moves between different modes.

The new test 'view3d_mesh_particle_edit_mode_simple'
in `tests/ui_simulate/test_undo.py` exposes both issues.

3 files changed, 17 insertions, 3 deletions

diff --git a/source/blender/editors/physics/CMakeLists.txt b/source/blender/editors/physics/CMakeLists.txt
index 2b9d9aaa0e1..a607663763e 100644
--- a/source/blender/editors/physics/CMakeLists.txt
+++ b/source/blender/editors/physics/CMakeLists.txt
@@ -25,6 +25,7 @@ set(INC
   ../../makesdna
   ../../makesrna
   ../../windowmanager
+  ../../../../intern/clog
   ../../../../intern/glew-mx
   ../../../../intern/guardedalloc
   ../../../../intern/mantaflow/extern
diff --git a/source/blender/editors/physics/particle_edit.c b/source/blender/editors/physics/particle_edit.c
index ec3eb9c6a3a..d0ebac82c90 100644
--- a/source/blender/editors/physics/particle_edit.c
+++ b/source/blender/editors/physics/particle_edit.c
@@ -5395,6 +5395,9 @@ static void free_all_psys_edit(Object *object)
 
 void ED_object_particle_edit_mode_enter_ex(Depsgraph *depsgraph, Scene *scene, Object *ob)
 {
+  /* Needed so #ParticleSystemModifierData.mesh_final is set. */
+  BKE_scene_graph_evaluated_ensure(depsgraph, G_MAIN);
+
   PTCacheEdit *edit;
 
   ob->mode |= OB_MODE_PARTICLE_EDIT;
diff --git a/source/blender/editors/physics/particle_edit_undo.c b/source/blender/editors/physics/particle_edit_undo.c
index 5d2e0e5b6ef..c764a5fd80f 100644
--- a/source/blender/editors/physics/particle_edit_undo.c
+++ b/source/blender/editors/physics/particle_edit_undo.c
@@ -27,6 +27,8 @@
 
 #include "MEM_guardedalloc.h"
 
+#include "CLG_log.h"
+
 #include "DNA_meshdata_types.h"
 #include "DNA_scene_types.h"
 #include "DNA_windowmanager_types.h"
@@ -44,11 +46,15 @@
 #include "ED_object.h"
 #include "ED_particle.h"
 #include "ED_physics.h"
+#include "ED_undo.h"
 
 #include "particle_edit_utildefines.h"
 
 #include "physics_intern.h"
 
+/** Only needed this locally. */
+static CLG_LogRef LOG = {"ed.undo.particle_edit"};
+
 /* -------------------------------------------------------------------- */
 /** \name Undo Conversion
  * \{ */
@@ -251,13 +257,13 @@ static void particle_undosys_step_decode(struct bContext *C,
                                          bool UNUSED(is_final))
 {
   Depsgraph *depsgraph = CTX_data_depsgraph_pointer(C);
-  /* TODO(campbell): undo_system: use low-level API to set mode. */
-  ED_object_mode_set_ex(C, OB_MODE_PARTICLE_EDIT, false, NULL);
-  BLI_assert(particle_undosys_poll(C));
 
   ParticleUndoStep *us = (ParticleUndoStep *)us_p;
   Scene *scene = us->scene_ref.ptr;
   Object *ob = us->object_ref.ptr;
+
+  ED_object_particle_edit_mode_enter_ex(depsgraph, scene, ob);
+
   PTCacheEdit *edit = PE_get_current(depsgraph, scene, ob);
   if (edit) {
     undoptcache_to_editcache(&us->data, edit);
@@ -267,6 +273,10 @@ static void particle_undosys_step_decode(struct bContext *C,
       BKE_particle_batch_cache_dirty_tag(edit->psys, BKE_PARTICLE_BATCH_DIRTY_ALL);
     }
     DEG_id_tag_update(&ob->id, ID_RECALC_GEOMETRY);
+
+    ED_undo_object_set_active_or_warn(scene, CTX_data_view_layer(C), ob, us_p->name, &LOG);
+
+    BLI_assert(particle_undosys_poll(C));
   }
   else {
     BLI_assert(0);