From 3a9ae7a0f28791e3ab451954c63d72142ad3ccf4 Mon Sep 17 00:00:00 2001
From: "Jeffrey C. Ollie" <jeff@ocjtech.us>
Date: Sun, 12 Apr 2026 16:04:16 -0500
Subject: [PATCH] decbkm: expose DECBKM to libghostty-vt

---
 include/ghostty/vt/key/encoder.h | 30 +++++++++++++++++++-----------
 src/terminal/c/key_encode.zig    |  7 +++++++
 2 files changed, 26 insertions(+), 11 deletions(-)

diff --git a/include/ghostty/vt/key/encoder.h b/include/ghostty/vt/key/encoder.h
index dc9e27e7e..3aeec6597 100644
--- a/include/ghostty/vt/key/encoder.h
+++ b/include/ghostty/vt/key/encoder.h
@@ -87,24 +87,32 @@ typedef enum GHOSTTY_ENUM_TYPED {
 typedef enum GHOSTTY_ENUM_TYPED {
     /** Terminal DEC mode 1: cursor key application mode (value: bool) */
     GHOSTTY_KEY_ENCODER_OPT_CURSOR_KEY_APPLICATION = 0,
-    
+
     /** Terminal DEC mode 66: keypad key application mode (value: bool) */
     GHOSTTY_KEY_ENCODER_OPT_KEYPAD_KEY_APPLICATION = 1,
-    
+
     /** Terminal DEC mode 1035: ignore keypad with numlock (value: bool) */
     GHOSTTY_KEY_ENCODER_OPT_IGNORE_KEYPAD_WITH_NUMLOCK = 2,
-    
+
     /** Terminal DEC mode 1036: alt sends escape prefix (value: bool) */
     GHOSTTY_KEY_ENCODER_OPT_ALT_ESC_PREFIX = 3,
-    
+
     /** xterm modifyOtherKeys mode 2 (value: bool) */
     GHOSTTY_KEY_ENCODER_OPT_MODIFY_OTHER_KEYS_STATE_2 = 4,
-    
+
     /** Kitty keyboard protocol flags (value: GhosttyKittyKeyFlags bitmask) */
     GHOSTTY_KEY_ENCODER_OPT_KITTY_FLAGS = 5,
-    
+
     /** macOS option-as-alt setting (value: GhosttyOptionAsAlt) */
     GHOSTTY_KEY_ENCODER_OPT_MACOS_OPTION_AS_ALT = 6,
+
+    /** Backarrow key mode (value: bool)
+     * See https://vt100.net/dec/ek-vt3xx-tp-002.pdf page 170
+     * If `false` (the default), `backspace` emits 0x7f
+     * If `true`, `backspace` emits 0x08
+     */
+    GHOSTTY_KEY_ENCODER_OPT_BACKARROW_KEY_MODE = 7,
+
     GHOSTTY_KEY_ENCODER_OPT_MAX_VALUE = GHOSTTY_ENUM_MAX_VALUE,
 } GhosttyKeyEncoderOption;
 
@@ -205,17 +213,17 @@ GHOSTTY_API void ghostty_key_encoder_setopt_from_terminal(GhosttyKeyEncoder enco
  * size_t required = 0;
  * GhosttyResult result = ghostty_key_encoder_encode(encoder, event, NULL, 0, &required);
  * assert(result == GHOSTTY_OUT_OF_SPACE);
- * 
+ *
  * // Allocate buffer of required size
  * char *buf = malloc(required);
- * 
+ *
  * // Encode with properly sized buffer
  * size_t written = 0;
  * result = ghostty_key_encoder_encode(encoder, event, buf, required, &written);
  * assert(result == GHOSTTY_SUCCESS);
- * 
+ *
  * // Use the encoded sequence...
- * 
+ *
  * free(buf);
  * @endcode
  *
@@ -226,7 +234,7 @@ GHOSTTY_API void ghostty_key_encoder_setopt_from_terminal(GhosttyKeyEncoder enco
  * char buf[128];
  * size_t written = 0;
  * GhosttyResult result = ghostty_key_encoder_encode(encoder, event, buf, sizeof(buf), &written);
- * 
+ *
  * if (result == GHOSTTY_SUCCESS) {
  *   // Write the encoded sequence to the terminal
  *   write(pty_fd, buf, written);
diff --git a/src/terminal/c/key_encode.zig b/src/terminal/c/key_encode.zig
index 15fa74dd8..f5d459f01 100644
--- a/src/terminal/c/key_encode.zig
+++ b/src/terminal/c/key_encode.zig
@@ -52,6 +52,11 @@ pub const Option = enum(c_int) {
     modify_other_keys_state_2 = 4,
     kitty_flags = 5,
     macos_option_as_alt = 6,
+    /// DEC Backarrow Key Mode (DECBKM)
+    /// See https://vt100.net/dec/ek-vt3xx-tp-002.pdf page 170
+    /// If `false` (the default), `backspace` emits 0x7f
+    /// If `true`, `backspace` emits 0x08
+    backarrow_key_mode = 7,
 
     /// Input type expected for setting the option.
     pub fn InType(comptime self: Option) type {
@@ -61,6 +66,7 @@ pub const Option = enum(c_int) {
             .ignore_keypad_with_numlock,
             .alt_esc_prefix,
             .modify_other_keys_state_2,
+            .backarrow_key_mode,
             => bool,
             .kitty_flags => u8,
             .macos_option_as_alt => OptionAsAlt,
@@ -114,6 +120,7 @@ fn setoptTyped(
             }
             opts.macos_option_as_alt = value.*;
         },
+        .backarrow_key_mode => opts.backarrow_key_mode = value.*,
     }
 }