From ca5a6813636ffdf3439158e08561ea842ba5c0b4 Mon Sep 17 00:00:00 2001
From: Daniel Imms <2193314+Tyriar@users.noreply.github.com>
Date: Sun, 28 Dec 2025 07:34:55 -0800
Subject: [PATCH] Add quirks setting to control DECSET/DECRST 12

Fixes #5314
---
 src/common/InputHandler.ts            | 12 ++++++++----
 src/common/services/OptionsService.ts |  3 ++-
 src/common/services/Services.ts       |  5 +++++
 typings/xterm.d.ts                    | 21 +++++++++++++++++++++
 4 files changed, 36 insertions(+), 5 deletions(-)

diff --git a/src/common/InputHandler.ts b/src/common/InputHandler.ts
index e6231725..99ae6252 100644
--- a/src/common/InputHandler.ts
+++ b/src/common/InputHandler.ts
@@ -1877,7 +1877,7 @@ export class InputHandler extends Disposable implements IInputHandler {
    * | 7     | Auto-wrap Mode (DECAWM).                                | #Y      |
    * | 8     | Auto-repeat Keys (DECARM). Always on.                   | #N      |
    * | 9     | X10 xterm mouse protocol.                               | #Y      |
-   * | 12    | Start Blinking Cursor.                                  | #Y      |
+   * | 12    | Start Blinking Cursor.                                  | #P[Requires the allowSetCursorBlink quirk option enabled.] |
    * | 25    | Show Cursor (DECTCEM).                                  | #Y      |
    * | 45    | Reverse wrap-around.                                    | #Y      |
    * | 47    | Use Alternate Screen Buffer.                            | #Y      |
@@ -1930,7 +1930,9 @@ export class InputHandler extends Disposable implements IInputHandler {
           this._coreService.decPrivateModes.wraparound = true;
           break;
         case 12:
-          this._optionsService.options.cursorBlink = true;
+          if (this._optionsService.rawOptions.quirks?.allowSetCursorBlink) {
+            this._optionsService.options.cursorBlink = true;
+          }
           break;
         case 45:
           this._coreService.decPrivateModes.reverseWraparound = true;
@@ -2125,7 +2127,7 @@ export class InputHandler extends Disposable implements IInputHandler {
    * | 7     | No Wraparound Mode (DECAWM).                            | #Y      |
    * | 8     | No Auto-repeat Keys (DECARM).                           | #N      |
    * | 9     | Don't send Mouse X & Y on button press.                 | #Y      |
-   * | 12    | Stop Blinking Cursor.                                   | #Y      |
+   * | 12    | Stop Blinking Cursor.                                   | #P[Requires the allowSetCursorBlink quirk option enabled.] |
    * | 25    | Hide Cursor (DECTCEM).                                  | #Y      |
    * | 45    | No reverse wrap-around.                                 | #Y      |
    * | 47    | Use Normal Screen Buffer.                               | #Y      |
@@ -2171,7 +2173,9 @@ export class InputHandler extends Disposable implements IInputHandler {
           this._coreService.decPrivateModes.wraparound = false;
           break;
         case 12:
-          this._optionsService.options.cursorBlink = false;
+          if (this._optionsService.rawOptions.quirks?.allowSetCursorBlink) {
+            this._optionsService.options.cursorBlink = false;
+          }
           break;
         case 45:
           this._coreService.decPrivateModes.reverseWraparound = false;
diff --git a/src/common/services/OptionsService.ts b/src/common/services/OptionsService.ts
index b33a0856..54c3db23 100644
--- a/src/common/services/OptionsService.ts
+++ b/src/common/services/OptionsService.ts
@@ -53,7 +53,8 @@ export const DEFAULT_OPTIONS: Readonly<Required<ITerminalOptions>> = {
   convertEol: false,
   termName: 'xterm',
   cancelEvents: false,
-  overviewRuler: {}
+  overviewRuler: {},
+  quirks: {}
 };
 
 const FONT_WEIGHT_OPTIONS: Extract<FontWeight, string>[] = ['normal', 'bold', '100', '200', '300', '400', '500', '600', '700', '800', '900'];
diff --git a/src/common/services/Services.ts b/src/common/services/Services.ts
index 849abcd5..a73d99f3 100644
--- a/src/common/services/Services.ts
+++ b/src/common/services/Services.ts
@@ -262,6 +262,7 @@ export interface ITerminalOptions {
   windowOptions?: IWindowOptions;
   wordSeparator?: string;
   overviewRuler?: IOverviewRulerOptions;
+  quirks?: ITerminalQuirks;
   scrollOnEraseInDisplay?: boolean;
 
   [key: string]: any;
@@ -300,6 +301,10 @@ export interface ITheme {
   extendedAnsi?: string[];
 }
 
+export interface ITerminalQuirks {
+  allowSetCursorBlink?: boolean;
+}
+
 export const IOscLinkService = createDecorator<IOscLinkService>('OscLinkService');
 export interface IOscLinkService {
   serviceBrand: undefined;
diff --git a/typings/xterm.d.ts b/typings/xterm.d.ts
index 8e34a00e..a5d82601 100644
--- a/typings/xterm.d.ts
+++ b/typings/xterm.d.ts
@@ -197,6 +197,12 @@ declare module '@xterm/xterm' {
      */
     minimumContrastRatio?: number;
 
+    /**
+     * Control various quirks features that are either non-standard or standard
+     * in but generally rejected in modern terminals.
+     */
+    quirks?: ITerminalQuirks;
+
     /**
      * Whether to reflow the line containing the cursor when the terminal is
      * resized. Defaults to false, because shells usually handle this
@@ -406,6 +412,21 @@ declare module '@xterm/xterm' {
     extendedAnsi?: string[];
   }
 
+  /**
+   * Control various quirks features that are either non-standard or standard
+   * in but generally rejected in modern terminals.
+   */
+  export interface ITerminalQuirks {
+    /**
+     * Enables support for DECSET 12 and DECRST 12 which controls cursor blink.
+     * Programs such as `vim` may use this to set the cursor blink state but may
+     * not change it back when exiting. Generally the terminal emulator should
+     * be in control of whether the cursor blinks or not and the application in
+     * modern terminals. Note that DECRQM works regardless of this option.
+     */
+    allowSetCursorBlink?: boolean;
+  }
+
   /**
    * Pty information for Windows.
    */