fix(csection): only keep cross-section curves for visible images

Cross-section panels kept one curve for every image ever shown, so plots
with hundreds of hidden images became progressively slower to refresh.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: PierreRaybaut

plotpy/panels/csection/csplot.py

@@ -167,13 +167,19 @@ def get_cross_section_curves(self) -> list[CrossSectionItem]:
     def items_changed(self, plot: BasePlot) -> None:
         """Items have changed in the plot
 
+        Cross section curves are kept for visible image items only: the curve
+        of an image item that is hidden or removed is deleted, and created
+        again when the item is shown. Keeping the curves of hidden items would
+        make every change cost more as images accumulate in the plot (e.g. an
+        application showing one image at a time among many hidden ones).
+
         Args:
             plot: Plot containing the image items to be used for cross section
         """
-        # Del obsolete cross section items
         new_sources = plot.get_items(item_type=ICSImageItemType)
+        # Del cross section items of removed or hidden image items
         for source in self.known_items.copy():
-            if source not in new_sources:
+            if source not in new_sources or not source.isVisible():
                 curve = self.known_items.pop(source)
                 curve.clear_data()  # useful to emit SIG_CS_CURVE_CHANGED
                 # (eventually notify other panels that the

plotpy/tests/unit/test_cross_section_visible_items.py

@@ -0,0 +1,73 @@
+# -*- coding: utf-8 -*-
+#
+# Licensed under the terms of the BSD 3-Clause
+# (see plotpy/LICENSE for details)
+
+"""Cross section panels keep curves for visible images only; unchanged axis
+styles do not lay out the plot again."""
+
+from __future__ import annotations
+
+import numpy as np
+from guidata.qthelpers import qt_app_context
+from qtpy import QtWidgets as QW
+
+from plotpy.builder import make
+
+
+def _window():
+    win = make.dialog(type="image", show_xsection=True, show_ysection=True)
+    win.show()
+    return win
+
+
+def test_cross_section_curves_follow_visible_images():
+    """Hidden images have no cross section curve, until they are shown again."""
+    with qt_app_context(exec_loop=False):
+        win = _window()
+        plot = win.manager.get_plot()
+        xcs = win.manager.get_xcs_panel().cs_plot
+        ycs = win.manager.get_ycs_panel().cs_plot
+        data = np.random.default_rng(0).random((16, 16))
+        images = []
+        for _index in range(20):
+            if images:
+                plot.set_item_visible(images[-1], False, notify=False, replot=False)
+            image = make.image(data.copy())
+            plot.add_item(image)
+            images.append(image)
+            QW.QApplication.processEvents()
+        assert list(xcs.known_items) == [images[-1]]
+        assert list(ycs.known_items) == [images[-1]]
+        plot.set_item_visible(images[-1], False)
+        assert not xcs.known_items
+        plot.set_item_visible(images[0], True)
+        assert list(xcs.known_items) == [images[0]]
+        assert xcs.known_items[images[0]].isVisible()
+        plot.del_item(images[0])
+        assert not xcs.known_items and not ycs.known_items
+        win.close()
+
+
+def test_unchanged_axis_style_does_not_relayout():
+    """Setting the same titles again neither lays out the plot nor notifies."""
+    with qt_app_context(exec_loop=False):
+        win = _window()
+        plot = win.manager.get_plot()
+        plot.set_titles(title="T", xlabel="x", ylabel="y", xunit="mm", yunit="mm")
+        layouts, notifications = [], []
+        update_layout = plot.updateLayout
+        plot.updateLayout = lambda: (layouts.append(1), update_layout())
+        plot.SIG_PLOT_LABELS_CHANGED.connect(notifications.append)
+        plot.set_titles(title="T", xlabel="x", ylabel="y", xunit="mm", yunit="mm")
+        assert not layouts and not notifications
+        plot.set_titles(xlabel="x2")
+        assert plot.get_axis_title("bottom") == "x2"
+        assert plot.axisTitle(plot.get_axis_id("bottom")).text() == "x2 (mm)"
+        assert layouts and notifications
+        win.close()
+
+
+if __name__ == "__main__":
+    test_cross_section_curves_follow_visible_images()
+    test_unchanged_axis_style_does_not_relayout()