fix: adding a beat could wipe a beat timeline after "Set amount in measure"

tests/timelines/beat/test_beat_timeline.py

@@ -1,5 +1,6 @@
 import pytest
 
+from tilia.timelines.beat.timeline import BeatTimeline
 from tilia.ui import commands
 
 
@@ -436,3 +437,27 @@ def test_crop_timeline_with_many_beats(self, resources, tilia, tilia_state):
 
         assert len(tilia.timelines[0]) == 11
         assert tilia.timelines[0][-1].get_data("time") == 10
+
+
+class TestGetExtensionFromBeatPattern:
+    def test_starting_measure_emptier_than_beat_pattern(self):
+        extension = BeatTimeline.get_extension_from_beat_pattern(
+            [4], 3, beats_on_starting_measure=2
+        )
+        assert extension == [2, 1]
+
+    def test_starting_measure_as_full_as_beat_pattern(self):
+        extension = BeatTimeline.get_extension_from_beat_pattern(
+            [4], 3, beats_on_starting_measure=4
+        )
+        assert extension == [3]
+
+    def test_starting_measure_fuller_than_beat_pattern(self):
+        """ "Set amount in measure" can put more beats in a measure than the
+        beat pattern prescribes for it. There is nothing left to fill in that
+        case, so the extension has to start a new measure.
+        """
+        extension = BeatTimeline.get_extension_from_beat_pattern(
+            [4], 3, beats_on_starting_measure=5
+        )
+        assert extension == [3]

tests/ui/timelines/beat/test_beat_timeline_ui.py

@@ -509,6 +509,29 @@ def test_updates_measure_numbers(self, beat_tlui):
 
         assert [get_displayed_measure_number(b) for b in beat_tlui] == ["1", "", "2"]
 
+    def test_add_beat_when_last_measure_is_fuller_than_beat_pattern(self, beat_tlui):
+        beat_tlui.timeline.beat_pattern = [4]
+        for i in range(24):
+            beat_tlui.create_beat(i / 10)
+
+        # give a middle measure one beat more than the beat pattern prescribes
+        beat_tlui.select_element(beat_tlui[16])
+        with Serve(Get.FROM_USER_INT, (True, 5)):
+            commands.execute("timeline.beat.set_amount_in_measure")
+
+        # then make that measure the last one
+        beat_tlui.deselect_all_elements()
+        for element in list(beat_tlui)[-3:]:
+            beat_tlui.select_element(element)
+        commands.execute("timeline.component.delete")
+        assert beat_tlui.timeline.beats_in_measure == [4, 4, 4, 4, 5]
+
+        commands.execute("media.seek", 3.0)
+        commands.execute("timeline.beat.add")
+
+        assert len(beat_tlui) == 22
+        assert beat_tlui.timeline.beats_in_measure == [4, 4, 4, 4, 5, 1]
+
 
 class TestFillWithBeats:
     @pytest.fixture(autouse=True)

tilia/timelines/beat/timeline.py

@@ -417,13 +417,13 @@ def get_extension_from_beat_pattern(
                 diff = beats - beats_on_starting_measure
                 extension = [min(diff, amount)]
                 remaining_beats = amount - diff
-            elif beats_on_starting_measure == beats:
+            else:
+                # A measure may hold more beats than the beat pattern
+                # prescribes for it, as "Set amount in measure" sets an
+                # arbitrary amount. There is nothing left to fill in that
+                # case, so the extension starts a new measure.
                 extension = []
                 remaining_beats = amount
-            else:
-                raise ValueError(
-                    "More beats on starting measure than found in the iterator"
-                )
         else:
             extension = []
             remaining_beats = amount
@@ -472,7 +472,7 @@ def extend_beats_in_measure(self, amount: int) -> None:
 
         bp_index = (self.measure_count % len(self.beat_pattern)) - 1
         is_last_measure_complete = (
-            self._beats_in_measure[-1] == self.beat_pattern[bp_index]
+            self._beats_in_measure[-1] >= self.beat_pattern[bp_index]
         )
 
         if is_last_measure_complete: