fix(frame): start the corner fade at the brightest palette stop

A ladder is a hue walk, not a brightness walk, so painting the hot end
backwards put the dim colour at the corner.  Sort the ramp by stock
brightness and follow the pane height for the bottom-left glow.

tests/test_frame.py

@@ -1,15 +1,17 @@
-"""The BBS frames: grey edges, corners fading white → grey → cyan."""
+"""The BBS frames: grey edges, and a short palette glow in two corners."""
 
 from __future__ import annotations
 
 from rich.text import Text
 
-from beefy_player.palettes import PALETTES
+from beefy_player.palettes import PALETTES, brightness
 from beefy_player.widgets import frame
 
 #: The original ladder, used as a fixed reference in these tests.
 STOPS = PALETTES["spectrum"]
-HOT = STOPS[-1]  # the corner colour
+#: Its glow: the brightest cell first, dimmer cells towards the edge.
+RAMP = frame.fade_ramp(STOPS)
+HOT = RAMP[0]  # the corner colour
 
 
 def styles_of(line: Text) -> list[tuple[str, str]]:
@@ -18,26 +20,29 @@ def styles_of(line: Text) -> list[tuple[str, str]]:
     ]
 
 
+def test_the_corner_gets_the_brightest_colour_of_the_fade() -> None:
+    ranks = [brightness(colour) for colour in RAMP]
+    assert ranks == sorted(ranks, reverse=True)
+    # spectrum's hot end is red, which is dimmer than the yellow before it:
+    # the corner must still be the bright one, not the ladder's last stop
+    assert STOPS[-1] != HOT
+    assert brightness(HOT) > brightness(STOPS[-1])
+    assert set(RAMP) <= set(STOPS)
+    assert frame.corner_fade(len(RAMP), STOPS) == frame.EDGE
+
+
 def test_only_the_top_right_corner_fades_on_the_top_edge() -> None:
     spans = styles_of(frame.top(30, stops=STOPS))
     assert spans[0] == ("╭", frame.EDGE)  # plain grey corner
-    # the fade runs into the top-right corner, hot end last
-    assert [style for _text, style in spans[-4:-1]] == [
-        STOPS[-4],
-        STOPS[-3],
-        STOPS[-2],
-    ]
+    # the fade runs into the top-right corner, brightest cell last
+    assert [style for _text, style in spans[-4:-1]] == [RAMP[3], RAMP[2], RAMP[1]]
     assert spans[-1] == ("╮", f"bold {HOT}")
 
 
 def test_only_the_bottom_left_corner_fades_on_the_bottom_edge() -> None:
     spans = styles_of(frame.bottom(30, STOPS))
     assert spans[0] == ("╰", f"bold {HOT}")
-    assert [style for _text, style in spans[1:4]] == [
-        STOPS[-2],
-        STOPS[-3],
-        STOPS[-4],
-    ]
+    assert [style for _text, style in spans[1:4]] == [RAMP[1], RAMP[2], RAMP[3]]
     assert spans[-1] == ("╯", frame.EDGE)  # plain grey corner
 
 
@@ -51,15 +56,15 @@ def test_the_sides_glow_towards_their_own_corner_only() -> None:
         frame.EDGE,
         frame.EDGE,
         frame.EDGE,
-        STOPS[-4],
-        STOPS[-3],
-        STOPS[-2],
+        RAMP[3],
+        RAMP[2],
+        RAMP[1],
     ]
     # right column fades down from the top-right corner
     assert [frame.side_style(row, 10, "right", STOPS) for row in range(10)] == [
-        STOPS[-2],
-        STOPS[-3],
-        STOPS[-4],
+        RAMP[1],
+        RAMP[2],
+        RAMP[3],
         frame.EDGE,
         frame.EDGE,
         frame.EDGE,
@@ -98,3 +103,16 @@ def test_content_wider_than_the_pane_is_trimmed_not_spilled() -> None:
     assert lines[1].endswith("│")
     # the original renderable is left untouched
     assert long_line.plain == "x" * 40
+
+
+def test_a_short_pane_still_glows_in_its_bottom_left_corner() -> None:
+    """The fade follows the pane's height, not the number of content rows."""
+    group = frame.wrap([Text("one line")], 20, 6, stops=STOPS)
+    lines = list(group.renderables)
+    # rows 1-4 are the content box; the fade climbs its last three rows
+    assert str(lines[1].spans[0].style) == frame.EDGE
+    assert [str(line.spans[0].style) for line in lines[2:5]] == [
+        RAMP[3],
+        RAMP[2],
+        RAMP[1],
+    ]

tests/test_palettes.py

@@ -13,6 +13,7 @@ from beefy_player.palettes import (
     DEFAULT_PALETTE,
     PALETTE_ORDER,
     PALETTES,
+    brightness,
     palette_stops,
 )
 from beefy_player.widgets import frame
@@ -133,13 +134,27 @@ async def test_the_spectrum_rows_use_the_active_palette() -> None:
         assert colours(spectrum._row_palette(12, 0, False)) <= set(PALETTES["neon"])
 
 
-def test_the_corner_fade_walks_back_down_the_ladder() -> None:
+def test_the_corner_fade_starts_at_the_brightest_stop() -> None:
     stops = PALETTES["spectrum"]
-    assert frame.corner_fade(0, stops) == stops[-1]
-    assert frame.corner_fade(1, stops) == stops[-2]
-    assert frame.corner_fade(3, stops) == stops[-4]
-    # the fade is short: past four cells it is just the border grey
-    assert frame.corner_fade(4, stops) == frame.EDGE
+    ramp = frame.fade_ramp(stops)
+    assert frame.corner_fade(0, stops) == ramp[0]
+    assert frame.corner_fade(1, stops) == ramp[1]
+    # the fade is short: past the ramp it is just the border grey
+    assert frame.corner_fade(len(ramp), stops) == frame.EDGE
+    # spectrum's hot end is red, which is dimmer than the yellow before it;
+    # the corner still gets the bright colour
+    assert ramp[0] != stops[-1]
+    assert ramp[-1] == stops[-1]
+
+
+def test_every_palette_fades_from_bright_to_dim() -> None:
+    """No palette may put a brighter cell further from the corner."""
+    for name, stops in PALETTES.items():
+        ramp = frame.fade_ramp(stops)
+        assert ramp, name
+        ranks = [brightness(colour) for colour in ramp]
+        assert ranks == sorted(ranks, reverse=True), (name, ramp)
+        assert set(ramp) <= set(stops), name
 
 
 @pytest.mark.asyncio