Fix duplicated arrow tips in DashedVMobject (issue #3220) (#4484)

* Fix duplicated arrow tips in DashedVMobject and add tests

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Benjamin Hackl <devel@benjamin-hackl.at>

Author: 3 people

manim/mobject/types/vectorized_mobject.py

@@ -2832,6 +2832,21 @@ def __init__(
         self.dashed_ratio = dashed_ratio
         self.num_dashes = num_dashes
         super().__init__(color=color, **kwargs)
+
+        # Work on a copy to avoid mutating the caller's mobject (e.g. removing tips).
+        base_vmobject = vmobject
+        vmobject = base_vmobject.copy()
+
+        # TipableVMobject instances (Arrow, Vector, etc.) carry tips as submobjects.
+        # When dashing such objects, each subcurve would otherwise include its own
+        # tip, leading to many overlapping arrowheads. Pop tips from the working
+        # copy and re-attach them only once after the dashes are created.
+        tips = None
+        if hasattr(vmobject, "pop_tips"):
+            popped_tips = vmobject.pop_tips()
+            if len(popped_tips.submobjects) > 0:
+                tips = popped_tips
+
         r = self.dashed_ratio
         n = self.num_dashes
         if n > 0:
@@ -2917,6 +2932,9 @@ def __init__(
         # Family is already taken care of by get_subcurve
         # implementation
         if config.renderer == RendererType.OPENGL:
-            self.match_style(vmobject, recurse=False)
+            self.match_style(base_vmobject, recurse=False)
         else:
-            self.match_style(vmobject, family=False)
+            self.match_style(base_vmobject, family=False)
+
+        if tips is not None:
+            self.add(*tips.submobjects)

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

@@ -0,0 +1,43 @@
+from manim import ORIGIN, UR, Arrow, DashedVMobject, VGroup
+from manim.mobject.geometry.tips import ArrowTip, StealthTip
+
+
+def _collect_tips(mobject):
+    return [mob for mob in mobject.get_family() if isinstance(mob, ArrowTip)]
+
+
+def test_dashed_arrow_has_single_tip():
+    dashed = DashedVMobject(Arrow(ORIGIN, 2 * UR))
+    tips = _collect_tips(dashed)
+
+    assert len(tips) == 1
+
+
+def test_dashed_arrow_tip_not_duplicated_in_group_opacity():
+    base_arrow = Arrow(ORIGIN, 2 * UR)
+    faded_arrow = base_arrow.copy().set_fill(opacity=0.4).set_stroke(opacity=0.4)
+
+    dashed_group = (
+        VGroup(DashedVMobject(faded_arrow))
+        .set_fill(opacity=0.4, family=True)
+        .set_stroke(opacity=0.4, family=True)
+    )
+
+    tips = _collect_tips(dashed_group)
+
+    assert len(tips) == 1
+
+
+def test_dashed_arrow_custom_tip_shape_has_single_tip():
+    dashed = DashedVMobject(Arrow(ORIGIN, 2 * UR, tip_shape=StealthTip))
+    tips = _collect_tips(dashed)
+
+    assert len(tips) == 1
+    assert isinstance(tips[0], StealthTip)
+
+
+def test_dashed_arrow_with_start_tip_has_two_tips():
+    dashed = DashedVMobject(Arrow(ORIGIN, 2 * UR).add_tip(at_start=True))
+    tips = _collect_tips(dashed)
+
+    assert len(tips) == 2