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.

beefy_player/palettes.py

@@ -75,6 +75,32 @@ PALETTES: dict[str, tuple[str, ...]] = {
     "mono": ("bright_black", "white", "bright_white"),
 }
 
+#: The stock names from dark to bright.  These are xterm's default values in
+#: Rec. 601 luma order — the closest thing to a stock answer.  A theme may
+#: disagree about the exact numbers, but the ordering is what the frame fades
+#: need: a ladder is a *hue* walk, so where it ends says nothing about which
+#: end is brighter.
+BRIGHTNESS: tuple[str, ...] = (
+    "black",
+    "blue",
+    "red",
+    "bright_red",
+    "magenta",
+    "bright_magenta",
+    "bright_blue",
+    "green",
+    "bright_black",
+    "cyan",
+    "bright_green",
+    "bright_cyan",
+    "yellow",
+    "white",
+    "bright_yellow",
+    "bright_white",
+)
+
+_RANK = {name: rank for rank, name in enumerate(BRIGHTNESS)}
+
 #: Cycle order for the palette key.
 PALETTE_ORDER: tuple[str, ...] = tuple(PALETTES)
 
@@ -86,3 +112,8 @@ def palette_stops(name: str | None) -> tuple[str, ...]:
     """The ladder for ``name``, falling back to the default palette."""
     key = (name or "").strip().lower()
     return PALETTES.get(key, PALETTES[DEFAULT_PALETTE])
+
+
+def brightness(colour: str) -> int:
+    """How bright a stock name is: 0 (black) to 15 (bright white)."""
+    return _RANK.get(colour.strip().lower(), 0)

beefy_player/widgets/frame.py

@@ -2,8 +2,9 @@
 
 The panes draw their own frames instead of using Textual's ``border`` so the
 corners can be coloured.  Two of them — the top right and the bottom left —
-run a short fade out of the palette's hot end and back into the border grey;
-everything else, including the other two corners, is that grey.
+run a short fade out of the palette's hot end and back into the border grey,
+brightest colour at the corner; everything else, including the other two
+corners, is that grey.
 
 The ladder is whatever palette the app is on (``c`` cycles them), which is why
 every function here takes ``stops`` rather than reading a module constant.
@@ -11,12 +12,14 @@ every function here takes ``stops`` rather than reading a module constant.
 
 from __future__ import annotations
 
+from functools import cache
+
 from rich.cells import cell_len
 from rich.console import Group, RenderableType
 from rich.text import Text
 from textual.widget import Widget
 
-from ..palettes import DEFAULT_PALETTE, PALETTES
+from ..palettes import DEFAULT_PALETTE, PALETTES, brightness
 from .common import active_stops
 
 #: The frame itself: one grey, the same grey as the scrollbar thumb.
@@ -26,20 +29,32 @@ EDGE = "bright_black"
 FADE_CELLS = 4
 
 
+@cache
+def fade_ramp(stops: tuple[str, ...]) -> tuple[str, ...]:
+    """The corner ramp: brightest cell first, border grey past its end.
+
+    The colours come from the palette's hot end, but a ladder is a hue walk,
+    not a brightness walk: ``spectrum`` ends on ``red``, which is dimmer than
+    the ``yellow`` just before it, so painting the ladder backwards put the
+    dim colour at the corner and the glow out on the edge.  Sorting the hot
+    end by brightness keeps the corner the brightest cell and dims outwards,
+    whatever theme sits behind the names.
+    """
+    hot = [name for name in stops if name != EDGE][-FADE_CELLS:]
+    return tuple(sorted(hot, key=brightness, reverse=True))
+
+
 def corner_fade(distance: int, stops: tuple[str, ...] = ()) -> str:
     """Colour ``distance`` cells out from a corner; 0 is the corner glyph.
 
-    The fade is short — four cells — and walks backwards down the palette from
-    its hot end, so the corner glows in the palette's brightest colour and then
-    hands over to the plain border grey.
+    The fade is short — four cells — and runs from the brightest colour the
+    palette's hot end offers back into the plain border grey.
     """
     if not stops:
         stops = PALETTES[DEFAULT_PALETTE]
+    ramp = fade_ramp(tuple(stops))
     distance = max(0, distance)
-    if distance >= FADE_CELLS:
-        return EDGE
-    index = len(stops) - 1 - distance
-    return stops[index] if index >= 0 else EDGE
+    return ramp[distance] if distance < len(ramp) else EDGE
 
 
 def inner_size(width: int, height: int) -> tuple[int, int]:
@@ -119,7 +134,6 @@ def wrap(
     width = max(4, width)
     height = max(2, height)
     inner_width, inner_height = inner_size(width, height)
-    rows = len(lines[:inner_height])
 
     framed: list[Text] = [top(width, title, stops)]
     for index, line in enumerate(lines[:inner_height]):
@@ -129,12 +143,12 @@ def wrap(
             line = line.copy()
             line.truncate(inner_width, overflow="ellipsis")
         row = Text(no_wrap=True)
-        row.append_text(side_row(index, rows, "left", stops))
+        row.append_text(side_row(index, inner_height, "left", stops))
         row.append_text(line)
         pad = inner_width - cell_len(line.plain)
         if pad > 0:
             row.append(" " * pad)
-        row.append_text(side_row(index, rows, "right", stops))
+        row.append_text(side_row(index, inner_height, "right", stops))
         framed.append(row)
     while len(framed) < height - 1:
         index = len(framed) - 1

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
 
 
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