From ca199ba13f7f400fb42705fe01f2bdb803d33ceb Mon Sep 17 00:00:00 2001
From: Campbell Barton <campbell@blender.org>
Date: Tue, 6 Aug 2024 21:18:40 +1000
Subject: [PATCH] Fix #125353: Select by active material fails to select the
 active slot

When (de)selecting the active material, use the active slot
falling back to the first-used slot (for non-active objects).

Resolve regression in [0].

Ref !125948

[0]: 296d05060d22714a5fe46af7206aa7066982e9c8
---
 source/blender/blenkernel/BKE_material.h        |  8 ++++++++
 source/blender/blenkernel/intern/material.cc    | 11 +++++++++++
 source/blender/editors/render/render_shading.cc |  3 ++-
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/source/blender/blenkernel/BKE_material.h b/source/blender/blenkernel/BKE_material.h
index 6ba7a9cc84a..c0600656784 100644
--- a/source/blender/blenkernel/BKE_material.h
+++ b/source/blender/blenkernel/BKE_material.h
@@ -115,6 +115,14 @@ bool BKE_object_material_slot_remove(struct Main *bmain, struct Object *ob);
 bool BKE_object_material_slot_used(struct Object *object, short actcol);
 
 int BKE_object_material_index_get(Object *ob, const Material *ma);
+/**
+ * A version of #BKE_object_material_index_get that takes an index to test first.
+ *
+ * \param hint_index: When this index is in a valid range, test it first.
+ * Useful when an active-index is preferred but may not match the material.
+ */
+int BKE_object_material_index_get_with_hint(Object *ob, const Material *ma, int hint_index);
+
 int BKE_object_material_ensure(Main *bmain, Object *ob, Material *material);
 
 struct Material *BKE_gpencil_material(struct Object *ob, short act);
diff --git a/source/blender/blenkernel/intern/material.cc b/source/blender/blenkernel/intern/material.cc
index d9c06ebeaa5..52faf2eabc5 100644
--- a/source/blender/blenkernel/intern/material.cc
+++ b/source/blender/blenkernel/intern/material.cc
@@ -844,6 +844,17 @@ int BKE_object_material_index_get(Object *ob, const Material *ma)
   return -1;
 }
 
+int BKE_object_material_index_get_with_hint(Object *ob, const Material *ma, const int hint_index)
+{
+  short *totcol = BKE_object_material_len_p(ob);
+  if ((hint_index >= 0) && (hint_index < *totcol)) {
+    if (ma == BKE_object_material_get(ob, hint_index + 1)) {
+      return hint_index;
+    }
+  }
+  return BKE_object_material_index_get(ob, ma);
+}
+
 int BKE_object_material_ensure(Main *bmain, Object *ob, Material *material)
 {
   if (!material) {
diff --git a/source/blender/editors/render/render_shading.cc b/source/blender/editors/render/render_shading.cc
index 4b7e9afae70..bb74cfd29d7 100644
--- a/source/blender/editors/render/render_shading.cc
+++ b/source/blender/editors/render/render_shading.cc
@@ -397,7 +397,8 @@ static int material_slot_de_select(bContext *C, bool select)
       continue;
     }
 
-    short mat_nr_active = BKE_object_material_index_get(ob, mat_active);
+    const short mat_nr_active = BKE_object_material_index_get_with_hint(
+        ob, mat_active, obact ? obact->actcol - 1 : -1);
 
     if (mat_nr_active == -1) {
       continue;