decbkm: address review comments

* Don't alter Kitty keyboard protocol responses. Kitty does not support
  DECBKM so KKP doesn't take DECBKM into consideration.
* Make better use of the function key lookup to control what sequence is
  returned when backspace is pressed using the legacy encoding.

src/input/function_keys.zig

@@ -43,6 +43,9 @@ pub const Entry = struct {
 
     /// The sequence to send to the pty if this entry matches.
     sequence: []const u8,
+
+    /// Sequence to send to the PTY if DECBKM is set.
+    sequence_decbkm: ?[]const u8 = null,
 };
 
 /// The list of modifier combinations for modify other key sequences.
@@ -161,8 +164,8 @@ pub const keys = keys: {
         .{ .mods = .{ .alt = true, .super = true, .ctrl = true }, .modify_other_keys = .set_other, .sequence = "\x1b[27;15;127~" },
         .{ .mods = .{ .alt = true, .super = true, .ctrl = true, .shift = true }, .modify_other_keys = .set_other, .sequence = "\x1b[27;16;127~" },
 
-        .{ .mods = .{ .ctrl = true }, .sequence = "\x08" },
-        .{ .sequence = "\x7f" },
+        .{ .mods = .{ .ctrl = true }, .sequence = "\x08", .sequence_decbkm = "\x7f" },
+        .{ .sequence = "\x7f", .sequence_decbkm = "\x08" },
     });
 
     result.set(.tab, &.{

src/input/key_encode.zig

@@ -189,9 +189,7 @@ fn kitty(
                 switch (event.key) {
                     .enter => return try writer.writeByte('\r'),
                     .tab => return try writer.writeByte('\t'),
-                    .backspace => return try writer.writeByte(
-                        if (opts.backarrow_key_mode) 0x08 else 0x7F,
-                    ),
+                    .backspace => return try writer.writeByte(0x7F),
                     else => {},
                 }
             }
@@ -664,7 +662,10 @@ fn pcStyleFunctionKey(
             continue;
         }
 
-        if (keyval == .backspace and backarrow_key_mode) return "\x08";
+        decbkm: {
+            if (!backarrow_key_mode) break :decbkm;
+            return entry.sequence_decbkm orelse break :decbkm;
+        }
 
         return entry.sequence;
     }
@@ -1267,13 +1268,13 @@ test "kitty: enter, backspace, tab" {
         try testing.expectEqualStrings("\x7f", writer.buffered());
     }
     {
-        // DECBKM set
+        // DECBKM set (Kitty does not support DECBKM so there should be no change)
         var writer: std.Io.Writer = .fixed(&buf);
         try kitty(&writer, .{ .key = .backspace, .mods = .{}, .utf8 = "" }, .{
             .kitty_flags = .{ .disambiguate = true },
             .backarrow_key_mode = true,
         });
-        try testing.expectEqualStrings("\x08", writer.buffered());
+        try testing.expectEqualStrings("\x7f", writer.buffered());
     }
     {
         var writer: std.Io.Writer = .fixed(&buf);
@@ -1912,7 +1913,26 @@ test "legacy: backspace with utf8 (dead key state)" {
     try testing.expectEqualStrings("", writer.buffered());
 }
 
+test "kitty: backspace (DECBKM reset) (report_all: true)" {
+    var buf: [128]u8 = undefined;
+    var writer: std.Io.Writer = .fixed(&buf);
+    try kitty(&writer, .{
+        .key = .backspace,
+    }, .{
+        .kitty_flags = .{
+            .disambiguate = true,
+            .report_events = true,
+            .report_alternates = true,
+            .report_all = true,
+            .report_associated = true,
+        },
+        .backarrow_key_mode = false,
+    });
+    try testing.expectEqualStrings("\x1b[127u", writer.buffered());
+}
+
 test "kitty: backspace (DECBKM set) (report_all: true)" {
+    // Kitty does not support DECBKM so there should be no difference.
     var buf: [128]u8 = undefined;
     var writer: std.Io.Writer = .fixed(&buf);
     try kitty(&writer, .{
@@ -2091,6 +2111,18 @@ test "legacy: backspace (DECBKM reset)" {
     try testing.expectEqualStrings("\x7f", writer.buffered());
 }
 
+test "legacy: backspace (DECBKM reset, with ctrl)" {
+    var buf: [128]u8 = undefined;
+    var writer: std.Io.Writer = .fixed(&buf);
+    try legacy(&writer, .{
+        .key = .backspace,
+        .mods = .{
+            .ctrl = true,
+        },
+    }, .{ .backarrow_key_mode = false });
+    try testing.expectEqualStrings("\x08", writer.buffered());
+}
+
 test "legacy: backspace (DECBKM set)" {
     var buf: [128]u8 = undefined;
     var writer: std.Io.Writer = .fixed(&buf);
@@ -2101,6 +2133,18 @@ test "legacy: backspace (DECBKM set)" {
     try testing.expectEqualStrings("\x08", writer.buffered());
 }
 
+test "legacy: backspace (DECBKM set, with ctrl)" {
+    var buf: [128]u8 = undefined;
+    var writer: std.Io.Writer = .fixed(&buf);
+    try legacy(&writer, .{
+        .key = .backspace,
+        .mods = .{
+            .ctrl = true,
+        },
+    }, .{ .backarrow_key_mode = true });
+    try testing.expectEqualStrings("\x7f", writer.buffered());
+}
+
 test "legacy: ctrl+shift+char with modify other state 2" {
     var buf: [128]u8 = undefined;
     var writer: std.Io.Writer = .fixed(&buf);