Preserve multi-panel figure size in visual-test compare()

`compare()` previously forced every figure into a single 5-inch canvas with
constrained layout. For multi-panel figures (e.g. scanpy-style color=[...] or
multiple coordinate systems) this crammed the panels together and pushed each
panel's per-axis colorbar/legend into the neighbouring panel.

Only normalize size + layout for single-panel figures (count gridspec-placed
axes; colorbar/legend insets have no subplotspec). Multi-panel figures keep
their own size and spacing; the thumbnail is produced by the existing resize
step. Single-panel baselines are unaffected; multi-panel baselines are
regenerated.

Author: timtreis

tests/conftest.py

@@ -503,18 +503,27 @@ def compare(cls, basename: str, tolerance: float | None = None):
 
         width, height = CANVAS_WIDTH, CANVAS_HEIGHT  # base dimensions; actual PNG may grow/shrink
         fig = plt.gcf()
-        fig.set_size_inches(width / DPI, height / DPI)
-        fig.set_dpi(DPI)
-
-        # Try to get a reasonable layout first (helps with axes/labels)
-        if not fig.get_constrained_layout():
-            try:
-                fig.set_layout_engine("constrained")
-            except (ValueError, RuntimeError):
+
+        # Count the real panels (gridspec-placed axes); colorbar/legend inset axes have no
+        # subplotspec. For single-panel figures we normalise size + layout as before. For
+        # multi-panel figures we keep the figure's own size and spacing, otherwise forcing the
+        # whole grid into a single-panel canvas squeezes each panel's colorbar/legend into the
+        # neighbouring panel. The small thumbnail is produced purely by the resize step below.
+        n_panels = sum(1 for sub_ax in fig.axes if sub_ax.get_subplotspec() is not None)
+
+        if n_panels <= 1:
+            fig.set_size_inches(width / DPI, height / DPI)
+            fig.set_dpi(DPI)
+
+            # Try to get a reasonable layout first (helps with axes/labels)
+            if not fig.get_constrained_layout():
                 try:
-                    fig.tight_layout(pad=2.0, rect=[0.02, 0.02, 0.98, 0.98])
+                    fig.set_layout_engine("constrained")
                 except (ValueError, RuntimeError):
-                    fig.subplots_adjust(left=0.1, right=0.9, top=0.9, bottom=0.1)
+                    try:
+                        fig.tight_layout(pad=2.0, rect=[0.02, 0.02, 0.98, 0.98])
+                    except (ValueError, RuntimeError):
+                        fig.subplots_adjust(left=0.1, right=0.9, top=0.9, bottom=0.1)
 
         plt.figure(fig.number)  # ensure this figure is current