fix(geometry): recompute DashedLine dashes on put_start_and_end_on

manim/mobject/geometry/line.py

@@ -327,6 +327,43 @@ def __init__(
         self.clear_points()
         self.add(*dashes)
 
+    def _reset_dashes_from_endpoints(
+        self, start: Point3DLike, end: Point3DLike
+    ) -> None:
+        underlying = Line(np.asarray(start, dtype=float), np.asarray(end, dtype=float))
+        underlying.match_style(self)
+        for attr in ("path_arc", "buff"):
+            if hasattr(self, attr):
+                setattr(underlying, attr, getattr(self, attr))
+        if underlying.path_arc != 0:
+            underlying.generate_points()
+        num_dashes = max(
+            2,
+            int(
+                np.ceil(
+                    (underlying.get_length() / self.dash_length) * self.dashed_ratio
+                )
+            ),
+        )
+        dashes = DashedVMobject(
+            underlying,
+            num_dashes=num_dashes,
+            dashed_ratio=self.dashed_ratio,
+        )
+        self.set_submobjects(list(dashes.submobjects))
+
+    def put_start_and_end_on(
+        self,
+        start: Point3DLike,
+        end: Point3DLike,
+    ) -> Self:
+        start_arr = np.asarray(start, dtype=float)
+        end_arr = np.asarray(end, dtype=float)
+        if np.allclose(start_arr, end_arr):
+            return super().put_start_and_end_on(start, end)
+        self._reset_dashes_from_endpoints(start, end)
+        return self
+
     def _calculate_num_dashes(self) -> int:
         """Returns the number of dashes in the dashed line.
 

tests/module/mobject/types/vectorized_mobject/test_dashed_vmobject.py

@@ -1,4 +1,6 @@
-from manim import ORIGIN, UR, Arrow, DashedLine, DashedVMobject, VGroup
+import pytest
+
+from manim import ORIGIN, RIGHT, UR, Arrow, DashedLine, DashedVMobject, VGroup
 from manim.mobject.geometry.tips import ArrowTip, StealthTip
 
 
@@ -57,3 +59,15 @@ def test_become_nonzero_to_zero_dashed_line_does_not_crash():
     normal = DashedLine(ORIGIN, 2 * UR)
     zero = DashedLine(ORIGIN, ORIGIN)
     normal.become(zero)
+
+
+def test_dashed_line_recomputes_dashes_on_put_start_and_end_on():
+    """Regression test for https://github.com/ManimCommunity/manim/issues/3989."""
+    line = DashedLine(ORIGIN, RIGHT)
+    initial_dash_length = line.submobjects[0].get_length()
+    initial_num_dashes = len(line.submobjects)
+    line.put_start_and_end_on(ORIGIN, 5 * RIGHT)
+    assert len(line.submobjects) > initial_num_dashes
+    assert line.submobjects[0].get_length() == pytest.approx(
+        initial_dash_length, rel=0.05
+    )