show names on tabs in tabbed layout

src/test.zig

@@ -3,6 +3,7 @@ const testing = std.testing;
 
 const act = @import("action");
 const input = @import("input");
+const font = @import("font.zig");
 const layout = @import("layout.zig");
 const Box = layout.Box;
 
@@ -422,3 +423,87 @@ test "map_to_output merges like any other setting" {
     const moved = merged.merge(.{ .map_to_output = "DP-2" });
     try testing.expectEqualStrings("DP-2", moved.map_to_output.?);
 }
+
+// ─── tab titles ──────────────────────────────────────────────────────────────
+
+test "a title that fits is drawn whole" {
+    const f = font.fit("foot", 10);
+    try testing.expectEqual(@as(usize, 4), f.end);
+    try testing.expectEqual(@as(usize, 4), f.cells);
+    try testing.expect(!f.elided);
+
+    // Exactly full is still whole: the ellipsis costs a cell, so only spend it
+    // when something is actually cut.
+    const exact = font.fit("foot", 4);
+    try testing.expectEqual(@as(usize, 4), exact.cells);
+    try testing.expect(!exact.elided);
+}
+
+test "a title too long for the tab loses its last cell to the ellipsis" {
+    const f = font.fit("firefox", 4);
+    try testing.expectEqualStrings("fir", "firefox"[0..f.end]);
+    try testing.expectEqual(@as(usize, 3), f.cells);
+    try testing.expect(f.elided);
+
+    // One cell of room leaves nothing but the ellipsis, and no room at all
+    // draws nothing rather than indexing past the end.
+    const one = font.fit("firefox", 1);
+    try testing.expectEqual(@as(usize, 0), one.end);
+    try testing.expect(one.elided);
+
+    const none = font.fit("firefox", 0);
+    try testing.expectEqual(@as(usize, 0), none.end);
+    try testing.expect(!none.elided);
+}
+
+test "titles are measured in characters, not bytes" {
+    // Three codepoints, six bytes: a tab four cells wide holds them all.
+    const f = font.fit("ünïx", 4);
+    try testing.expectEqual(@as(usize, 4), f.cells);
+    try testing.expect(!f.elided);
+
+    // And a cut lands on a codepoint boundary, never mid-sequence.
+    const cut = font.fit("ünïx", 3);
+    try testing.expect(cut.elided);
+    try testing.expectEqualStrings("ün", "ünïx"[0..cut.end]);
+}
+
+test "malformed utf-8 in a title yields one glyph per bad byte" {
+    var it: font.Iterator = .{ .bytes = "a\xffb\xc3" };
+    try testing.expectEqual(@as(?u21, 'a'), it.next());
+    try testing.expectEqual(@as(?u21, 0xfffd), it.next());
+    try testing.expectEqual(@as(?u21, 'b'), it.next());
+    // A truncated sequence at the end must terminate the walk, not loop.
+    try testing.expectEqual(@as(?u21, 0xfffd), it.next());
+    try testing.expectEqual(@as(?u21, null), it.next());
+}
+
+test "text measuring and cell fitting agree" {
+    const scale = 2;
+    // Whatever cellsForWidth says fits must actually fit.
+    var width: i32 = 0;
+    while (width < 200) : (width += 1) {
+        const cells = font.cellsForWidth(width, scale);
+        try testing.expect(font.textWidth(cells, scale) <= width);
+        try testing.expect(font.textWidth(cells + 1, scale) > width);
+    }
+}
+
+test "printable ascii all have glyphs, and unknown codepoints fall back" {
+    // The space is blank, and nothing else printable is.
+    var c: u8 = 0x21;
+    while (c <= 0x7e) : (c += 1) {
+        const rows = font.glyph(c);
+        var lit = false;
+        for (rows) |r| lit = lit or r != 0;
+        try testing.expect(lit);
+        // Nothing may spill outside the five column cell.
+        for (rows) |r| try testing.expectEqual(@as(u8, 0), r & ~@as(u8, 0b11111));
+    }
+    for (font.glyph(' ')) |r| try testing.expectEqual(@as(u8, 0), r);
+
+    // Control characters stay blank; other codepoints get the fallback box.
+    for (font.glyph('\n')) |r| try testing.expectEqual(@as(u8, 0), r);
+    try testing.expect(!std.mem.eql(u8, font.glyph('?'), font.glyph('あ')));
+    try testing.expectEqualSlices(u8, font.glyph(0xfffd), font.glyph('あ'));
+}