From 7e681b0bc02b98e932a51e5469bcaeb6649c5f75 Mon Sep 17 00:00:00 2001
From: Adam <2363879+adamdotdevin@users.noreply.github.com>
Date: Fri, 20 Feb 2026 07:38:05 -0600
Subject: fix(app): large text pasted into prompt-input causes main thread lock

---
 .../app/src/components/prompt-input/attachments.ts | 22 +++++++++++++++++++++-
 .../src/components/prompt-input/editor-dom.test.ts | 22 ++++++++++++++++++++++
 .../app/src/components/prompt-input/editor-dom.ts  | 15 +++++++++++++++
 3 files changed, 58 insertions(+), 1 deletion(-)

(limited to 'packages/app/src/components/prompt-input')

diff --git a/packages/app/src/components/prompt-input/attachments.ts b/packages/app/src/components/prompt-input/attachments.ts
index 9ea2e62a6..a9e4e4965 100644
--- a/packages/app/src/components/prompt-input/attachments.ts
+++ b/packages/app/src/components/prompt-input/attachments.ts
@@ -7,6 +7,19 @@ import { getCursorPosition } from "./editor-dom"
 
 export const ACCEPTED_IMAGE_TYPES = ["image/png", "image/jpeg", "image/gif", "image/webp"]
 export const ACCEPTED_FILE_TYPES = [...ACCEPTED_IMAGE_TYPES, "application/pdf"]
+const LARGE_PASTE_CHARS = 8000
+const LARGE_PASTE_BREAKS = 120
+
+function largePaste(text: string) {
+  if (text.length >= LARGE_PASTE_CHARS) return true
+  let breaks = 0
+  for (const char of text) {
+    if (char !== "\n") continue
+    breaks += 1
+    if (breaks >= LARGE_PASTE_BREAKS) return true
+  }
+  return false
+}
 
 type PromptAttachmentsInput = {
   editor: () => HTMLDivElement | undefined
@@ -14,7 +27,7 @@ type PromptAttachmentsInput = {
   isDialogActive: () => boolean
   setDraggingType: (type: "image" | "@mention" | null) => void
   focusEditor: () => void
-  addPart: (part: ContentPart) => void
+  addPart: (part: ContentPart) => boolean
   readClipboardImage?: () => Promise<File | null>
 }
 
@@ -89,6 +102,13 @@ export function createPromptAttachments(input: PromptAttachmentsInput) {
     }
 
     if (!plainText) return
+
+    if (largePaste(plainText)) {
+      if (input.addPart({ type: "text", content: plainText, start: 0, end: 0 })) return
+      input.focusEditor()
+      if (input.addPart({ type: "text", content: plainText, start: 0, end: 0 })) return
+    }
+
     const inserted = typeof document.execCommand === "function" && document.execCommand("insertText", false, plainText)
     if (inserted) return
 
diff --git a/packages/app/src/components/prompt-input/editor-dom.test.ts b/packages/app/src/components/prompt-input/editor-dom.test.ts
index 15e759f44..3088522a5 100644
--- a/packages/app/src/components/prompt-input/editor-dom.test.ts
+++ b/packages/app/src/components/prompt-input/editor-dom.test.ts
@@ -24,6 +24,28 @@ describe("prompt-input editor dom", () => {
     expect((container.childNodes[1] as HTMLElement).tagName).toBe("BR")
   })
 
+  test("createTextFragment avoids break-node explosion for large multiline content", () => {
+    const content = Array.from({ length: 220 }, () => "line").join("\n")
+    const fragment = createTextFragment(content)
+    const container = document.createElement("div")
+    container.appendChild(fragment)
+
+    expect(container.childNodes.length).toBe(1)
+    expect(container.childNodes[0]?.nodeType).toBe(Node.TEXT_NODE)
+    expect(container.textContent).toBe(content)
+  })
+
+  test("createTextFragment keeps terminal break in large multiline fallback", () => {
+    const content = `${Array.from({ length: 220 }, () => "line").join("\n")}\n`
+    const fragment = createTextFragment(content)
+    const container = document.createElement("div")
+    container.appendChild(fragment)
+
+    expect(container.childNodes.length).toBe(2)
+    expect(container.childNodes[0]?.textContent).toBe(content.slice(0, -1))
+    expect((container.childNodes[1] as HTMLElement).tagName).toBe("BR")
+  })
+
   test("length helpers treat breaks as one char and ignore zero-width chars", () => {
     const container = document.createElement("div")
     container.appendChild(document.createTextNode("ab\u200B"))
diff --git a/packages/app/src/components/prompt-input/editor-dom.ts b/packages/app/src/components/prompt-input/editor-dom.ts
index 4850a26ec..8575140d7 100644
--- a/packages/app/src/components/prompt-input/editor-dom.ts
+++ b/packages/app/src/components/prompt-input/editor-dom.ts
@@ -1,5 +1,20 @@
+const MAX_BREAKS = 200
+
 export function createTextFragment(content: string): DocumentFragment {
   const fragment = document.createDocumentFragment()
+  let breaks = 0
+  for (const char of content) {
+    if (char !== "\n") continue
+    breaks += 1
+    if (breaks > MAX_BREAKS) {
+      const tail = content.endsWith("\n")
+      const text = tail ? content.slice(0, -1) : content
+      if (text) fragment.appendChild(document.createTextNode(text))
+      if (tail) fragment.appendChild(document.createElement("br"))
+      return fragment
+    }
+  }
+
   const segments = content.split("\n")
   segments.forEach((segment, index) => {
     if (segment) {
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