commit 948a7c9 (HEAD -> blender2.8)
Author: Brecht Van Lommel <brechtvanlommel@gmail.com>
Date:   Thu Nov 1 14:18:01 2018 +0100

    Fix T57316: material preview icons not updating.
    
    The editors update callback was getting the COW datablock, it should be the
    original where the preview icons are stored.

diff --git a/source/blender/depsgraph/intern/eval/deg_eval_flush.cc b/source/blender/depsgraph/intern/eval/deg_eval_flush.cc
index 18806f3..e215941 100644
--- a/source/blender/depsgraph/intern/eval/deg_eval_flush.cc
+++ b/source/blender/depsgraph/intern/eval/deg_eval_flush.cc
@@ -262,13 +262,16 @@ void flush_editors_id_update(Main *bmain,
                DEG_DEBUG_PRINTF((::Depsgraph *)graph,
                                 EVAL, "Accumulated recalc bits for %s: %u\n",
                                 id_orig->name, (unsigned int)id_cow->recalc);
+
+               /* Callback for editors to update (icon previews). */
+               deg_editors_id_update(update_ctx, id_orig);
+               /* ID may need to get its auto-override operations refreshed. */
+               if (ID_IS_STATIC_OVERRIDE_AUTO(id_orig)) {
+                       id_orig->tag |= LIB_TAG_OVERRIDESTATIC_AUTOREFRESH;
+               }
+
                /* Inform editors. */
                if (deg_copy_on_write_is_expanded(id_cow)) {
-                       deg_editors_id_update(update_ctx, id_cow);
-                       /* ID may need to get its auto-override operations refreshed. */
-                       if (ID_IS_STATIC_OVERRIDE_AUTO(id_orig)) {
-                               id_orig->tag |= LIB_TAG_OVERRIDESTATIC_AUTOREFRESH;
-                       }
                        /* Inform draw engines that something was changed. */
                        flush_engine_data_update(id_cow);
                }
diff --git a/source/blender/makesrna/intern/rna_material.c b/source/blender/makesrna/intern/rna_material.c
index a842513..24baf0e 100644
--- a/source/blender/makesrna/intern/rna_material.c
+++ b/source/blender/makesrna/intern/rna_material.c
@@ -114,27 +114,8 @@ static void rna_Material_update_previews(Main *UNUSED(bmain), Scene *UNUSED(scen
 static void rna_MaterialGpencil_update(Main *bmain, Scene *scene, PointerRNA *ptr)
 {
        Material *ma = ptr->id.data;
-       PreviewImage *preview = ma->preview;
 
        rna_Material_update(bmain, scene, ptr);
-
-       /* update previews (icon and thumbnail) */
-       if (preview != NULL) {
-               bool changed = false;
-               if ((preview->flag[ICON_SIZE_ICON] & PRV_CHANGED) == 0) {
-                       preview->flag[ICON_SIZE_ICON] |= PRV_CHANGED;
-                       changed = true;
-               }
-
-               if ((preview->flag[ICON_SIZE_PREVIEW] & PRV_CHANGED) == 0) {
-                       preview->flag[ICON_SIZE_PREVIEW] |= PRV_CHANGED;
-                       changed = true;
-               }
-
-               if (changed) {
-                       WM_main_add_notifier(NC_MATERIAL | ND_SHADING_PREVIEW, ma);
-               }
-       }
        WM_main_add_notifier(NC_GPENCIL | ND_DATA, ma);
 }
 
@@ -143,7 +124,6 @@ static void rna_MaterialGpencil_nopreview_update(Main *bmain, Scene *scene, Poin
        Material *ma = ptr->id.data;
 
        rna_Material_update(bmain, scene, ptr);
-
        WM_main_add_notifier(NC_GPENCIL | ND_DATA, ma);
 }