v0.47.0: no album art shows a debossed panel, not a flat square

The sidebar's art square, when a track has no art, is now the list's own
background with a rounded panel set into it, debossed like the visualizer
and the now-playing panel. paint_deboss takes its rims in pixels when
asked, so a panel as tall as the sidebar is recessed as much as a short
one rather than fading over 16 px.

tests/test_round80.py

@@ -98,3 +98,46 @@ def test_thumb_paints(qapp):
     trough = image.pixelColor(7, 5)
     middle = image.pixelColor(7, thumb.center().y())
     assert middle != trough
+
+
+# --------------------------------------------------------------------------
+# no album art: a debossed panel, not a flat square
+# --------------------------------------------------------------------------
+
+def test_no_art_is_a_debossed_panel(qapp):
+    from PyQt6.QtCore import QObject, pyqtSignal
+    from lintunes.gui.sidebar import NO_ART_MARGIN, SidebarArt
+
+    class Player(QObject):
+        track_changed = pyqtSignal(object)
+
+    art = SidebarArt(Player())
+    art.resize(200, 200)
+    image = art.grab().toImage()
+    base = art.palette().base().color()
+    # the margin around the panel is the list's own background...
+    assert image.pixelColor(1, 100) == base
+    # ...the corner is rounded off, so it's background too...
+    assert image.pixelColor(NO_ART_MARGIN + 1, NO_ART_MARGIN + 1) == base
+    # ...and the panel is shaded darker at the top than in the middle
+    top = image.pixelColor(100, NO_ART_MARGIN + 2).lightness()
+    middle = image.pixelColor(100, 100).lightness()
+    assert top < middle
+
+
+def test_deboss_rims_can_be_fixed_in_pixels(qapp):
+    """On a 400 px panel the default rims are 32 px deep; 3 px rims leave
+    row 10 already on the face, a good deal lighter."""
+    from PyQt6.QtCore import QRectF
+    from PyQt6.QtGui import QImage, QPainter
+    color = QColor("#e0e0e0")
+
+    def row10(**kw):
+        image = QImage(100, 400, QImage.Format.Format_ARGB32)
+        image.fill(0)
+        painter = QPainter(image)
+        theme.paint_deboss(painter, QRectF(0.5, 0.5, 99, 399), color, 8, **kw)
+        painter.end()
+        return image.pixelColor(50, 10).lightness()
+
+    assert row10(rim=3) > row10() + 5