fix(tui): ignore unsupported Kitty CSI-u modifiers closes #1807

packages/tui/CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Fixed
 
 - Fixed TUI width calculation for regional indicator symbols (e.g. partial flag sequences like `🇨` during streaming) to prevent wrap drift and stale character artifacts in differential rendering.
+- Fixed Kitty CSI-u handling to ignore unsupported modifiers so modifier-only events do not insert stray printable characters ([#1807](https://github.com/badlogic/pi-mono/issues/1807))
 
 ## [0.55.4] - 2026-03-02
 

packages/tui/src/components/editor.ts

@@ -96,6 +96,8 @@ const KITTY_CSI_U_REGEX = /^\x1b\[(\d+)(?::(\d*))?(?::(\d+))?(?:;(\d+))?(?::(\d+
 const KITTY_MOD_SHIFT = 1;
 const KITTY_MOD_ALT = 2;
 const KITTY_MOD_CTRL = 4;
+const KITTY_LOCK_MASK = 64 + 128; // Caps Lock + Num Lock
+const KITTY_ALLOWED_MODIFIERS = KITTY_MOD_SHIFT | KITTY_LOCK_MASK;
 
 // Decode a printable CSI-u sequence, preferring the shifted key when present.
 function decodeKittyPrintable(data: string): string | undefined {
@@ -111,7 +113,10 @@ function decodeKittyPrintable(data: string): string | undefined {
 	// Modifiers are 1-indexed in CSI-u; normalize to our bitmask.
 	const modifier = Number.isFinite(modValue) ? modValue - 1 : 0;
 
-	// Ignore CSI-u sequences used for Alt/Ctrl shortcuts.
+	// Only accept printable CSI-u input for plain or Shift-modified text keys.
+	// Reject unsupported modifier bits (e.g. Super/Meta) to avoid inserting
+	// characters from modifier-only terminal events.
+	if ((modifier & ~KITTY_ALLOWED_MODIFIERS) !== 0) return undefined;
 	if (modifier & (KITTY_MOD_ALT | KITTY_MOD_CTRL)) return undefined;
 
 	// Prefer the shifted keycode when Shift is held.

packages/tui/src/keys.ts

@@ -1048,6 +1048,8 @@ export function parseKey(data: string): string | undefined {
 		const { codepoint, baseLayoutKey, modifier } = kitty;
 		const mods: string[] = [];
 		const effectiveMod = modifier & ~LOCK_MASK;
+		const supportedModifierMask = MODIFIERS.shift | MODIFIERS.ctrl | MODIFIERS.alt;
+		if ((effectiveMod & ~supportedModifierMask) !== 0) return undefined;
 		if (effectiveMod & MODIFIERS.shift) mods.push("shift");
 		if (effectiveMod & MODIFIERS.ctrl) mods.push("ctrl");
 		if (effectiveMod & MODIFIERS.alt) mods.push("alt");

packages/tui/test/editor.test.ts

@@ -361,6 +361,16 @@ describe("Editor component", () => {
 		});
 	});
 
+	describe("Kitty CSI-u handling", () => {
+		it("ignores printable CSI-u sequences with unsupported modifiers", () => {
+			const editor = new Editor(createTestTUI(), defaultEditorTheme);
+
+			editor.handleInput("\x1b[99;9u");
+
+			assert.strictEqual(editor.getText(), "");
+		});
+	});
+
 	describe("Unicode text editing behavior", () => {
 		it("inserts mixed ASCII, umlauts, and emojis as literal text", () => {
 			const editor = new Editor(createTestTUI(), defaultEditorTheme);

packages/tui/test/keys.test.ts

@@ -294,6 +294,12 @@ describe("parseKey", () => {
 			assert.strictEqual(parseKey(latinCtrlC), "ctrl+c");
 			setKittyProtocolActive(false);
 		});
+
+		it("should ignore Kitty CSI-u with unsupported modifiers", () => {
+			setKittyProtocolActive(true);
+			assert.strictEqual(parseKey("\x1b[99;9u"), undefined);
+			setKittyProtocolActive(false);
+		});
 	});
 
 	describe("Legacy key parsing", () => {