fix(plot2d): a capped rectangle is born at the cap, not half the image

add_rectangle_widget's default size is half the image, so a widget created with
max_extent appeared huge and then snapped down to the cap the first time it was
used — which reads as a bug ("the ROI is always huge and then it clamps down").
The default now honours max_extent per axis. An explicit w/h still wins, and an
uncapped widget is unchanged.

Author: CSSFrancis

anyplotlib/plot2d/_plot2d.py

@@ -1620,11 +1620,25 @@ def add_rectangle_widget(self, x: float | None = None, y: float | None = None,
         (the dragged corner pins, the opposite corner stays put).
         """
         iw, ih = self._state["image_width"], self._state["image_height"]
+        # The DEFAULT size honours max_extent. Without this a capped rectangle is
+        # born at half the image and then snaps down to the cap on first use —
+        # the ROI visibly appears huge and collapses, which looks like a bug.
+        def _cap(axis_default, cap):
+            return axis_default if cap is None else min(axis_default, float(cap))
+
+        if max_extent is None:
+            cap_w = cap_h = None
+        elif isinstance(max_extent, (tuple, list)):
+            cap_w, cap_h = max_extent[0], max_extent[1]
+        else:
+            cap_w = cap_h = max_extent
+        def_w = _cap(iw * 0.5, cap_w)
+        def_h = _cap(ih * 0.5, cap_h)
         widget = RectangleWidget(lambda: None,
                                  x=float(x) if x is not None else iw * 0.25,
                                  y=float(y) if y is not None else ih * 0.25,
-                                 w=float(w) if w is not None else iw * 0.5,
-                                 h=float(h) if h is not None else ih * 0.5,
+                                 w=float(w) if w is not None else def_w,
+                                 h=float(h) if h is not None else def_h,
                                  color=color, linewidth=linewidth,
                                  show_handles=show_handles,
                                  max_extent=max_extent)

anyplotlib/tests/test_interactive/test_widget_max_extent.py

@@ -91,6 +91,45 @@ def test_add_rectangle_widget_passes_max_extent(self):
         assert w.max_w == 8.0 and w.max_h == 12.0
 
 
+class TestDefaultSizeHonoursTheCap:
+    """A capped rectangle must be BORN at the cap.
+
+    The default size is half the image, so a capped ROI used to appear huge and
+    then snap down to the cap on first use — which reads as a bug ("the ROI is
+    always huge and then it clamps down")."""
+
+    def test_default_size_is_capped(self):
+        fig, ax = apl.subplots(1, 1)
+        v = ax.imshow(np.zeros((256, 256), dtype=np.float32))
+        w = v.add_rectangle_widget(max_extent=16.0)
+        assert w.w == 16.0 and w.h == 16.0
+
+    def test_default_size_respects_a_per_axis_cap(self):
+        fig, ax = apl.subplots(1, 1)
+        v = ax.imshow(np.zeros((256, 256), dtype=np.float32))
+        w = v.add_rectangle_widget(max_extent=(8, 32))
+        assert w.w == 8.0 and w.h == 32.0
+
+    def test_uncapped_default_is_unchanged(self):
+        fig, ax = apl.subplots(1, 1)
+        v = ax.imshow(np.zeros((256, 256), dtype=np.float32))
+        w = v.add_rectangle_widget()
+        assert w.w == 128.0 and w.h == 128.0      # half the image, as before
+
+    def test_explicit_size_wins_over_the_cap_default(self):
+        """An explicit w/h is the caller's business; the cap only bounds drags."""
+        fig, ax = apl.subplots(1, 1)
+        v = ax.imshow(np.zeros((256, 256), dtype=np.float32))
+        w = v.add_rectangle_widget(w=4, h=4, max_extent=16.0)
+        assert w.w == 4.0 and w.h == 4.0
+
+    def test_cap_larger_than_the_image_does_not_inflate_the_default(self):
+        fig, ax = apl.subplots(1, 1)
+        v = ax.imshow(np.zeros((32, 32), dtype=np.float32))
+        w = v.add_rectangle_widget(max_extent=1000.0)
+        assert w.w == 16.0 and w.h == 16.0        # still half the image
+
+
 # ═══════════════════════════════════════════════════════════════════════════
 # 2. Drag behaviour in the browser — the logic lives in figure_esm.js, so a
 #    Python-only test would prove nothing about what the user experiences.