Cover style "halves": left and right half blocks only

Twice the horizontal detail of "cells", for terminals such as macOS
Terminal whose upper and lower half blocks do not meet at the middle of
the cell.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019reK4DUGYqDUjniWYJvwUW

internal/ui/cover.go

@@ -99,6 +99,10 @@ var (
 	// Coarse, but exact in terminals that draw block elements from the font
 	// with the halves off the cell's middle (macOS Terminal).
 	cellsStyle = coverStyle{"cells", 1, 1, func(int) rune { return ' ' }}
+	// halvesStyle (2×1 pixels): left and right half blocks only. Twice the
+	// detail of cells across, for terminals whose upper and lower halves do not
+	// meet at the middle but whose left and right ones do.
+	halvesStyle = coverStyle{"halves", 2, 1, func(mask int) rune { return [4]rune{' ', '▌', '▐', '█'}[mask] }}
 )
 
 // quadrants[mask] is the glyph that fills the quadrants set in mask
@@ -141,6 +145,8 @@ func coverStyleFor(setting string, env func(string) string) coverStyle {
 		return quadrantStyle
 	case "cells":
 		return cellsStyle
+	case "halves":
+		return halvesStyle
 	}
 	// Terminal.app draws block elements from the font, and their halves do
 	// not meet at the cell's middle: rows of a poster come out uneven.

internal/ui/cover_test.go

@@ -277,6 +277,7 @@ func TestCoverStyleFor(t *testing.T) {
 		{"sextant", nil, "sextant"},
 		{"auto", map[string]string{"TERM": "xterm-256color", "TERM_PROGRAM": "Apple_Terminal"}, "cells"},
 		{"cells", map[string]string{"TERM": "xterm-kitty"}, "cells"},
+		{"halves", map[string]string{"TERM_PROGRAM": "Apple_Terminal"}, "halves"},
 		{"quadrant", map[string]string{"TERM_PROGRAM": "Apple_Terminal"}, "quadrant"},
 	} {
 		if got := coverStyleFor(tc.setting, env(tc.env)).name; got != tc.want {
@@ -358,3 +359,11 @@ func TestCellsStyleIsFlat(t *testing.T) {
 		t.Errorf("got %q %v %v", ch, fg, bg)
 	}
 }
+
+func TestHalvesStyle(t *testing.T) {
+	dark, light := color.RGBA{0, 0, 0, 255}, color.RGBA{250, 250, 250, 255}
+	ch, fg, bg := halvesStyle.bestCell([]color.RGBA{light, dark})
+	if ch != '▌' || fg != light || bg != dark {
+		t.Errorf("light left half: %q %v %v", ch, fg, bg)
+	}
+}

internal/ui/settings.go

@@ -26,7 +26,7 @@ type settingsValues struct {
 	notify, bell                                      bool
 }
 
-var coverStyles = []string{"auto", "kitty", "sextant", "quadrant", "cells"}
+var coverStyles = []string{"auto", "kitty", "sextant", "quadrant", "halves", "cells"}
 
 // settingsFrom reads the settings in use.
 func settingsFrom(c *config.Config) settingsValues {