Return the viewport extent in scene units so zoom survives recompute

docs/changelog.rst

@@ -39,6 +39,12 @@ Unreleased
   large atoms swung towards the eye by rotation no longer clip at the
   viewport edge, and a cell with no atoms is sized correctly too.
 
+- In the interactive viewer, changing the frame or a display style no
+  longer resets the zoom level.
+
+- A view opened with a zoom above one now starts zoomed in at that
+  level, rather than fitting the window regardless of the zoom set.
+
 0.20.0
 ------
 

src/hofmann/rendering/interactive.py

@@ -65,7 +65,7 @@ def _rotation_z(angle: float) -> np.ndarray:
 
 _KEY_ROTATION_STEP = 0.05  # radians (~3 degrees) per key press
 _KEY_ZOOM_FACTOR = 1.1  # multiplicative zoom per key press / scroll step
-_KEY_PAN_FRACTION = 0.05  # fraction of scene extent per key press
+_KEY_PAN_FRACTION = 0.05  # fraction of the viewport panned per key press
 _PERSPECTIVE_STEP = 0.1  # perspective increment per key press
 _PERSPECTIVE_FLOOR = 1e-9  # below this, the descent lands on Orthographic
 _DISTANCE_FACTOR = 1.05  # viewing distance multiplier per key press

src/hofmann/rendering/projection.py

@@ -69,7 +69,11 @@ def _scene_extent(
     Returns the radius of a 2D bounding circle centred at the origin
     that encloses every atom and unit-cell corner under any rotation:
     the largest centre distance plus display radius, widened by the
-    projection's worst-case magnification, and scaled by zoom.
+    projection's worst-case magnification.
+
+    The extent is in scene units and excludes ``view.zoom``: the
+    projected coordinates already carry it, so consumers must not
+    re-apply it.
     """
     coords = scene.frames[frame_index].coords
     dists = np.linalg.norm(coords - view.centre, axis=1)
@@ -104,7 +108,7 @@ def _scene_extent(
     # not a magic-threshold clamp on the magnification.
     max_extent *= view.projection.max_magnification(max_extent)
 
-    return float(max_extent * view.zoom)
+    return float(max_extent)
 
 
 def _make_wedges(

tests/test_rendering/test_interactive.py

@@ -201,6 +201,24 @@ def test_pan_down(self):
         expected = old_centre + step * view.rotation[1]
         np.testing.assert_allclose(view.centre, expected)
 
+    def test_pan_step_halves_when_zoom_doubles(self):
+        """The pan step is a fraction of the *visible* extent, so doubling
+        the zoom halves the world-space step.  Guards the ``/ view.zoom``
+        in the pan step (a ratio, not the production formula restated)."""
+        v1, s1, st1, iv1 = _key_action_fixtures()
+        v1.zoom = 1.0
+        c1 = v1.centre.copy()
+        _do_key("shift+left", v1, s1, st1, iv1, base_extent=10.0)
+        step_at_1 = float(np.linalg.norm(v1.centre - c1))
+
+        v2, s2, st2, iv2 = _key_action_fixtures()
+        v2.zoom = 2.0
+        c2 = v2.centre.copy()
+        _do_key("shift+left", v2, s2, st2, iv2, base_extent=10.0)
+        step_at_2 = float(np.linalg.norm(v2.centre - c2))
+
+        np.testing.assert_allclose(step_at_2, step_at_1 / 2.0)
+
     # -- Perspective --
 
     def test_perspective_increase(self):

tests/test_rendering/test_projection.py

@@ -202,6 +202,25 @@ def test_empty_scene_perspective_is_magnified(self):
         extent = _scene_extent(scene, view, 0, atom_scale=0.5)
         assert extent > 1.0  # pre-fix: gated out, returns the 1.0 floor
 
+    def test_extent_is_independent_of_zoom(self):
+        # The extent is in scene units; the coordinates carry the zoom.
+        # A frame-navigation recompute after zooming therefore returns
+        # the same viewport, so the interactive zoom is not reset.
+        scene = StructureScene(
+            species=["C"],
+            frames=[Frame(coords=np.array([[0.0, 0.0, 5.0]]))],
+            atom_styles={"C": AtomStyle(1.0, (0.5, 0.5, 0.5))},
+        )
+        e1 = _scene_extent(
+            scene, ViewState(zoom=1.0, projection=Orthographic()), 0,
+            atom_scale=0.5,
+        )
+        e2 = _scene_extent(
+            scene, ViewState(zoom=2.0, projection=Orthographic()), 0,
+            atom_scale=0.5,
+        )
+        assert e1 == e2
+
 
 class TestMakeWedges:
     def test_pure_composition_returns_single_full_circle(self):