From 4c0067c9ee0143b1629288f971fc55a0e1999424 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Sat, 19 Sep 2026 02:33:02 +0000
Subject: [PATCH 1/2] Initial plan


From f7af25026d00dc2afc660beeb3981568bfffd421 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Sat, 19 Sep 2026 02:49:04 +0000
Subject: [PATCH 2/2] Fix MIDIMessageEvent.data incorrectly typed as nullable

Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>
---
 tsc/internal/bundled/libs/lib.dom.d.ts        |  2 +-
 .../midiMessageEventDataNotNullable.js        | 17 +++++++++++++
 .../midiMessageEventDataNotNullable.symbols   | 21 ++++++++++++++++
 .../midiMessageEventDataNotNullable.types     | 24 +++++++++++++++++++
 tsc/testdata/fixtures/lib/dom.generated.d.ts  |  2 +-
 .../midiMessageEventDataNotNullable.ts        |  9 +++++++
 6 files changed, 73 insertions(+), 2 deletions(-)
 create mode 100644 tsc/testdata/baselines/reference/compiler/midiMessageEventDataNotNullable.js
 create mode 100644 tsc/testdata/baselines/reference/compiler/midiMessageEventDataNotNullable.symbols
 create mode 100644 tsc/testdata/baselines/reference/compiler/midiMessageEventDataNotNullable.types
 create mode 100644 tsc/testdata/tests/cases/compiler/midiMessageEventDataNotNullable.ts

diff --git a/tsc/internal/bundled/libs/lib.dom.d.ts b/tsc/internal/bundled/libs/lib.dom.d.ts
index 9e127c3f5a524..67374324d3886 100644
--- a/tsc/internal/bundled/libs/lib.dom.d.ts
+++ b/tsc/internal/bundled/libs/lib.dom.d.ts
@@ -23616,7 +23616,7 @@ interface MIDIMessageEvent extends Event {
      *
      * [MDN Reference](https://developer.mozilla.org/docs/Web/API/MIDIMessageEvent/data)
      */
-    readonly data: Uint8Array<ArrayBuffer> | null;
+    readonly data: Uint8Array<ArrayBuffer>;
 }
 
 declare var MIDIMessageEvent: {
diff --git a/tsc/testdata/baselines/reference/compiler/midiMessageEventDataNotNullable.js b/tsc/testdata/baselines/reference/compiler/midiMessageEventDataNotNullable.js
new file mode 100644
index 0000000000000..1141d43b29b5b
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/midiMessageEventDataNotNullable.js
@@ -0,0 +1,17 @@
+//// [tests/cases/compiler/midiMessageEventDataNotNullable.ts] ////
+
+//// [midiMessageEventDataNotNullable.ts]
+declare const input: MIDIInput;
+
+input.onmidimessage = (event) => {
+    // `data` should not be nullable, so this should not error.
+    event.data[0];
+};
+
+
+//// [midiMessageEventDataNotNullable.js]
+"use strict";
+input.onmidimessage = (event) => {
+    // `data` should not be nullable, so this should not error.
+    event.data[0];
+};
diff --git a/tsc/testdata/baselines/reference/compiler/midiMessageEventDataNotNullable.symbols b/tsc/testdata/baselines/reference/compiler/midiMessageEventDataNotNullable.symbols
new file mode 100644
index 0000000000000..d664677819caf
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/midiMessageEventDataNotNullable.symbols
@@ -0,0 +1,21 @@
+//// [tests/cases/compiler/midiMessageEventDataNotNullable.ts] ////
+
+=== midiMessageEventDataNotNullable.ts ===
+declare const input: MIDIInput;
+>input : Symbol(input, Decl(midiMessageEventDataNotNullable.ts, 0, 13))
+>MIDIInput : Symbol(MIDIInput, Decl(lib.dom.d.ts, --, --), Decl(lib.dom.d.ts, --, --))
+
+input.onmidimessage = (event) => {
+>input.onmidimessage : Symbol(MIDIInput.onmidimessage, Decl(lib.dom.d.ts, --, --))
+>input : Symbol(input, Decl(midiMessageEventDataNotNullable.ts, 0, 13))
+>onmidimessage : Symbol(MIDIInput.onmidimessage, Decl(lib.dom.d.ts, --, --))
+>event : Symbol(event, Decl(midiMessageEventDataNotNullable.ts, 2, 23))
+
+    // `data` should not be nullable, so this should not error.
+    event.data[0];
+>event.data : Symbol(MIDIMessageEvent.data, Decl(lib.dom.d.ts, --, --))
+>event : Symbol(event, Decl(midiMessageEventDataNotNullable.ts, 2, 23))
+>data : Symbol(MIDIMessageEvent.data, Decl(lib.dom.d.ts, --, --))
+
+};
+
diff --git a/tsc/testdata/baselines/reference/compiler/midiMessageEventDataNotNullable.types b/tsc/testdata/baselines/reference/compiler/midiMessageEventDataNotNullable.types
new file mode 100644
index 0000000000000..2852561ed1454
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/midiMessageEventDataNotNullable.types
@@ -0,0 +1,24 @@
+//// [tests/cases/compiler/midiMessageEventDataNotNullable.ts] ////
+
+=== midiMessageEventDataNotNullable.ts ===
+declare const input: MIDIInput;
+>input : MIDIInput
+
+input.onmidimessage = (event) => {
+>input.onmidimessage = (event) => {    // `data` should not be nullable, so this should not error.    event.data[0];} : (this: MIDIInput, event: MIDIMessageEvent) => void
+>input.onmidimessage : ((this: MIDIInput, ev: MIDIMessageEvent) => any) | null
+>input : MIDIInput
+>onmidimessage : ((this: MIDIInput, ev: MIDIMessageEvent) => any) | null
+>(event) => {    // `data` should not be nullable, so this should not error.    event.data[0];} : (this: MIDIInput, event: MIDIMessageEvent) => void
+>event : MIDIMessageEvent
+
+    // `data` should not be nullable, so this should not error.
+    event.data[0];
+>event.data[0] : number
+>event.data : Uint8Array<ArrayBuffer>
+>event : MIDIMessageEvent
+>data : Uint8Array<ArrayBuffer>
+>0 : 0
+
+};
+
diff --git a/tsc/testdata/fixtures/lib/dom.generated.d.ts b/tsc/testdata/fixtures/lib/dom.generated.d.ts
index d667879250cf4..ee99317bf27a5 100644
--- a/tsc/testdata/fixtures/lib/dom.generated.d.ts
+++ b/tsc/testdata/fixtures/lib/dom.generated.d.ts
@@ -23600,7 +23600,7 @@ interface MIDIMessageEvent extends Event {
      *
      * [MDN Reference](https://developer.mozilla.org/docs/Web/API/MIDIMessageEvent/data)
      */
-    readonly data: Uint8Array<ArrayBuffer> | null;
+    readonly data: Uint8Array<ArrayBuffer>;
 }
 
 declare var MIDIMessageEvent: {
diff --git a/tsc/testdata/tests/cases/compiler/midiMessageEventDataNotNullable.ts b/tsc/testdata/tests/cases/compiler/midiMessageEventDataNotNullable.ts
new file mode 100644
index 0000000000000..a7d9bfe601b50
--- /dev/null
+++ b/tsc/testdata/tests/cases/compiler/midiMessageEventDataNotNullable.ts
@@ -0,0 +1,9 @@
+// @strict: true
+// @lib: dom,es2015
+
+declare const input: MIDIInput;
+
+input.onmidimessage = (event) => {
+    // `data` should not be nullable, so this should not error.
+    event.data[0];
+};