fixed the stroke width issue with single color in streamlines (ManimC…

…ommunity#3436)

* fixed the stroke width issue with single color in streamlines

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

for more information, see https://pre-commit.ci

* Added test for streamlines

* Added test for streamlines

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: MrDiver <[email protected]>

manim/mobject/vector_field.py

@@ -829,7 +829,9 @@ def outside_box(p):
             step = max(1, int(len(points) / self.max_anchors_per_line))
             line.set_points_smoothly(points[::step])
             if self.single_color:
-                line.set_stroke(self.color)
+                line.set_stroke(
+                    color=self.color, width=self.stroke_width, opacity=opacity
+                )
             else:
                 if config.renderer == RendererType.OPENGL:
                     # scaled for compatibility with cairo

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

@@ -2,6 +2,7 @@
 
 import manim.utils.color as C
 from manim import VMobject
+from manim.mobject.vector_field import StreamLines
 
 
 def test_stroke_props_in_ctor():
@@ -24,3 +25,17 @@ def test_set_background_stroke():
     assert m.background_stroke_width == 2
     assert m.background_stroke_opacity == 0.8
     assert m.background_stroke_color.to_hex() == C.ORANGE.to_hex()
+
+
+def test_streamline_attributes_for_single_color():
+    vector_field = StreamLines(
+        lambda x: x,  # It is not important what this function is.
+        x_range=[-1, 1, 0.1],
+        y_range=[-1, 1, 0.1],
+        padding=0.1,
+        stroke_width=1.0,
+        opacity=0.2,
+        color=C.BLUE_D,
+    )
+    assert vector_field[0].stroke_width == 1.0
+    assert vector_field[0].stroke_opacity == 0.2