So far just a port of #119669 to latest main.

@@ -962,0 +1043,4 @@
+   * ensuring that at least one layer exists above.
+   *
+   * TODO: revisit this once those semantics are hammered out. */
+  if (layer == nullptr) {

@@ -977,2 +1106,3 @@
 
-  AnimData *adt = BKE_animdata_from_id(id);
+  bAction *action = id_action_ensure(bmain, id);
+  if (action == nullptr) {

@@ -370,0 +389,4 @@
+   *
+   * \return the index of the strip, or -1 if no strip was found at that time.
+   */
+  int strip_index_at_time(float time) const;

@@ -962,0 +1046,4 @@
+  if (layer == nullptr) {
+    /* TODO: count the rna paths properly (e.g. accounting for multi-element
+     * properties). */
+    combined_result.add(SingleKeyingResult::NO_VALID_LAYER, rna_paths.size());

@@ -119,0 +120,4 @@
+   * If the Action is empty, create a default layer with a single infinite
+   * keyframe strip.
+   */
+  void ensure_layer();

@@ -502,2 +534,3 @@
                                      float2 time_value,
-                                     const KeyframeSettings &settings);
+                                     const KeyframeSettings &settings,
+                                     const eInsertKeyFlags insert_key_flags);

@@ -64,0 +79,4 @@
+  /**
+   * Increase the count of the given `SingleKeyingResult` by `count`.
+   */
+  void add(SingleKeyingResult result, int count);

@@ -184,1 +184,4 @@
 
+void Action::ensure_layer()
+{
+  if (this->layers().size() == 0) {

@@ -185,0 +186,4 @@
+{
+  if (this->layers().size() == 0) {
+    /* TODO: default layer name localization. */
+    Layer &layer = this->layer_add("Layer");

@@ -397,0 +406,4 @@
+Layer *Action::get_layer_for_keyframing()
+{
+  /* TODO: handle multiple layers. */
+  if (this->layers().size() == 0) {

@@ -853,0 +902,4 @@
+   * allow. */
+  FCurve *fcurve = key_insertion_may_create_fcurve(insert_key_flags) ?
+                       &this->fcurve_find_or_create(binding, rna_path, array_index) :
+                       this->fcurve_find(binding, rna_path, array_index);

@@ -148,1 +153,4 @@
 
+  if (this->get_count(SingleKeyingResult::NO_VALID_LAYER) > 0) {
+    const int error_count = this->get_count(SingleKeyingResult::NO_VALID_LAYER);
+    errors.append(fmt::format(RPT_("Inserting keys on {:d} ID(s) has been skipped because there "

@@ -978,0 +1024,4 @@
+    return SingleKeyingResult::NO_VALID_STRIP;
+  }
+
+  /* TODO: morph key data based on Layer position in stack and Strip offset. */

@@ -978,0 +1045,4 @@
+  Binding *binding = action.binding_for_handle(binding_handle);
+  if (binding == nullptr) {
+    binding = &action.binding_add();
+    const bool success = action.assign_id(binding, *id);

@@ -978,0 +1053,4 @@
+  }
+
+  action.ensure_layer();
+  Layer *layer = action.get_layer_for_keyframing();

@@ -978,0 +1079,4 @@
+    BLI_assert(rna_path_id_to_prop.has_value());
+    Vector<float> rna_values = get_keyframe_values(&ptr, prop, use_visual_keyframing);
+
+    for (int property_index : rna_values.index_range()) {

@@ -978,0 +1018,4 @@
+  if (strip == nullptr) {
+    return SingleKeyingResult::NO_VALID_STRIP;
+  }
+  BLI_assert(strip->contains_frame(key_data.position.x));

@@ -978,0 +1042,4 @@
+  Binding *binding = action.binding_for_handle(binding_handle);
+  if (binding == nullptr) {
+    binding = &action.binding_add();
+    const bool success = action.assign_id(binding, *id);

@@ -118,0 +155,4 @@
+  /* The action has a binding and it's assigned to the object. */
+  ASSERT_EQ(1, action.bindings().size());
+  Binding *binding = action.binding(0);
+  EXPECT_EQ(object->adt->binding_handle, binding->handle);

@@ -118,0 +159,4 @@
+
+  /* We have the default layer and strip. */
+  ASSERT_TRUE(action.is_action_layered());
+  ASSERT_EQ(1, action.layers().size());

@@ -118,0 +172,4 @@
+
+  /* The fcurves in the channel bag are what we expect. */
+  EXPECT_EQ(1, channel_bag->fcurves().size());
+  const FCurve *fcurve = channel_bag->fcurve_find("rotation_mode", 0);

@@ -118,0 +411,4 @@
+
+  /* The action has a binding and it's assigned to the first object. */
+  ASSERT_EQ(1, action.bindings().size());
+  Binding *binding_1 = action.binding_for_handle(object->adt->binding_handle);

@@ -118,0 +426,4 @@
+  ASSERT_NE(nullptr, channel_bag_1);
+
+  /* Assign the action to the second object. */
+  armature_object->adt = BKE_animdata_ensure_id(&armature_object->id);

@@ -118,0 +441,4 @@
+  EXPECT_EQ(1, result_2.get_count(SingleKeyingResult::SUCCESS));
+
+  ASSERT_EQ(2, action.bindings().size());
+  Binding *binding_2 = action.binding_for_handle(armature_object->adt->binding_handle);

@@ -132,2 +132,3 @@
+      binding, "location", 0, {1.0f, 47.0f}, settings, INSERTKEY_NOFLAGS);
   another_strip.as<KeyframeStrip>().keyframe_insert(
-      binding, "location", 0, {1.0f, 47.0f}, settings);
+      binding, "location", 0, {1.0f, 47.0f}, settings, INSERTKEY_NOFLAGS);

@@ -118,0 +139,4 @@
+   * - Infinite KeyframeStrip
+   * - FCurve with a single key
+   */
+  object->empty_drawsize = 1.0;

@@ -118,0 +156,4 @@
+   * to the object. */
+  ASSERT_EQ(1, action.bindings().size());
+  Binding *binding = action.binding(0);
+  EXPECT_EQ(0, strcmp(object->id.name, binding->name));