Add visual tests for scalebar frameon/sizing/fixed-value options

Cover three orthogonal scalebar knobs that users routinely tune:
- frameon=False (no surrounding box)
- length_fraction + pad + border_pad (compact footprint)
- fixed_value + label (pin bar length, override displayed text)

Also drops a transient explanatory comment block above the non-visual
scalebar tests; the test names and docstrings are self-explanatory.

Author: timtreis

tests/pl/test_show.py

@@ -54,6 +54,27 @@ def test_plot_scalebar_styled(self, sdata_blobs: SpatialData):
             scalebar_params={"location": "lower right", "color": "white", "box_alpha": 0.6},
         )
 
+    def test_plot_scalebar_no_frame(self, sdata_blobs: SpatialData):
+        """Visual test: frameon=False drops the surrounding box."""
+        sdata_blobs.pl.render_images(element="blobs_image").pl.show(
+            scalebar_dx=1.0,
+            scalebar_params={"frameon": False, "color": "white"},
+        )
+
+    def test_plot_scalebar_compact(self, sdata_blobs: SpatialData):
+        """Visual test: padding and length_fraction shrink the scalebar footprint."""
+        sdata_blobs.pl.render_images(element="blobs_image").pl.show(
+            scalebar_dx=1.0,
+            scalebar_params={"length_fraction": 0.15, "pad": 0.1, "border_pad": 0.1},
+        )
+
+    def test_plot_scalebar_fixed_value_label(self, sdata_blobs: SpatialData):
+        """Visual test: fixed_value pins the bar length and label overrides the displayed text."""
+        sdata_blobs.pl.render_images(element="blobs_image").pl.show(
+            scalebar_dx=1.0,
+            scalebar_params={"fixed_value": 200, "label": "200 um"},
+        )
+
     def test_plot_user_ax_dpi_preserved(self, sdata_blobs: SpatialData):
         """Visual test: low DPI produces visibly pixelated rasterization (regression for #310).
 
@@ -158,11 +179,6 @@ def test_dpi_default_used_when_no_ax(sdata_blobs: SpatialData):
     plt.close(fig)
 
 
-# Scalebar regression tests (#614). The scalebar machinery existed but was
-# unreachable: show() did not accept scalebar_dx/scalebar_units. These tests
-# lock the wiring, the defaults, the kwargs passthrough, and validation.
-
-
 def _scalebars_on(ax):
     from matplotlib_scalebar.scalebar import ScaleBar