fix(terminal): skip paste EL after a same-batch alt-screen exit

Remember the pre-write alternate buffer so a paste echo and ESC[?1049l
in one chunk cannot inject ESC[K] into the restored normal screen.

Co-authored-by: 陈大猫 <binaricat@users.noreply.github.com>

components/terminal/runtime/terminalUserPaste.test.ts

@@ -448,6 +448,68 @@ test("long multi-line paste clears cursor-right residue after terminal echo", ()
   assert.deepEqual(writes, ["\x1b[K"]);
 });
 
+test("long multi-line paste does not erase to end of line on the alternate screen", () => {
+  const writes: string[] = [];
+  let bufferType: "alternate" | "normal" = "alternate";
+  const term = {
+    cols: 20,
+    rows: 4,
+    buffer: {
+      active: {
+        get type() {
+          return bufferType;
+        },
+      },
+    },
+    paste: () => {},
+    scrollToBottom: () => {},
+    write: (data: string) => writes.push(data),
+  };
+
+  const longPaste = Array.from({ length: 20 }, (_, index) => `line ${index} with enough content`).join("\n");
+  pasteTextIntoTerminal(term, longPaste, {
+    scrollOnPaste: false,
+  });
+  prepareTerminalDataForUserPasteDisplay(term, "\x1b[7mline 3 with enough content\x1b[27m");
+
+  assert.equal(clearPasteResidualAfterTerminalWrite(term), null);
+  assert.deepEqual(writes, []);
+
+  bufferType = "normal";
+  assert.equal(clearPasteResidualAfterTerminalWrite(term), null);
+  assert.deepEqual(writes, []);
+});
+
+test("long multi-line paste does not erase the restored normal buffer when the same batch leaves the alternate screen", () => {
+  const writes: string[] = [];
+  let bufferType: "alternate" | "normal" = "alternate";
+  const term = {
+    cols: 20,
+    rows: 4,
+    buffer: {
+      active: {
+        get type() {
+          return bufferType;
+        },
+      },
+    },
+    paste: () => {},
+    scrollToBottom: () => {},
+    write: (data: string) => writes.push(data),
+  };
+
+  const longPaste = Array.from({ length: 20 }, (_, index) => `line ${index} with enough content`).join("\n");
+  pasteTextIntoTerminal(term, longPaste, {
+    scrollOnPaste: false,
+  });
+  prepareTerminalDataForUserPasteDisplay(term, "\x1b[7mline 3 with enough content\x1b[27m\x1b[?1049l");
+  // xterm applies ESC[?1049l before the write callback observes buffer.active.
+  bufferType = "normal";
+
+  assert.equal(clearPasteResidualAfterTerminalWrite(term), null);
+  assert.deepEqual(writes, []);
+});
+
 test("sensitive paste override matches sanitized data containing zero-width characters", () => {
   const term = {
     paste: () => {},

components/terminal/runtime/terminalUserPaste.ts

@@ -32,6 +32,8 @@ type PasteDisplayState = {
   clearPending: number;
   pasteEchoFragments: string[];
   inPasteEchoActiveRegion: boolean;
+  // Buffer type observed while preparing this chunk, before xterm applies it.
+  preparedOnAlternateScreen: boolean;
 };
 
 type PasteInputScrollState = {
@@ -328,6 +330,7 @@ export function pasteTextIntoTerminal(
       clearPending: 0,
       pasteEchoFragments: getPasteEchoFragments(text),
       inPasteEchoActiveRegion: false,
+      preparedOnAlternateScreen: false,
     });
   }
 
@@ -521,10 +524,17 @@ function consumePasteInputState(
   return true;
 }
 
+const isAlternateScreenBuffer = (term: object): boolean =>
+  (term as { buffer?: { active?: { type?: string } } }).buffer?.active?.type === "alternate";
+
 export function prepareTerminalDataForUserPasteDisplay(term: object, data: string): string {
   const state = pasteDisplayStates.get(term);
   if (!isStateActive(state)) return data;
 
+  // Captured before term.write. The residual check runs in the write callback,
+  // after a same-batch ESC[?1049l has already restored the normal buffer.
+  state.preparedOnAlternateScreen = isAlternateScreenBuffer(term);
+
   const strippedActiveRegion = stripMatchedReadlineActiveRegion(data, state);
   if (strippedActiveRegion.matched) {
     state.clearPending = Math.max(state.clearPending, 3);
@@ -541,6 +551,16 @@ export function prepareTerminalDataForUserPasteDisplay(term: object, data: strin
 export function clearPasteResidualAfterTerminalWrite(term: object): string | null {
   const state = pasteDisplayStates.get(term);
   if (!isStateActive(state)) return null;
+  // Vim and other alternate-screen apps often end a paste chunk with the cursor
+  // mid-line. EL would erase the rest of that row, and the app may not redraw it (#3166).
+  // Drop the pending clear so it cannot fire after returning to the normal screen.
+  // preparedOnAlternateScreen covers a same-batch exit: buffer.active is already
+  // normal when this write callback runs.
+  if (state.preparedOnAlternateScreen || isAlternateScreenBuffer(term)) {
+    state.preparedOnAlternateScreen = false;
+    state.clearPending = 0;
+    return null;
+  }
   if (state.clearPending <= 0) return null;
   if (typeof (term as Partial<Pick<XTerm, "write">>).write !== "function") return null;