Fix repl cursor multiline overflow

lib/internal/readline/interface.js

@@ -16,6 +16,7 @@ const {
   MathFloor,
   MathMax,
   MathMaxApply,
+  MathMin,
   NumberIsFinite,
   ObjectDefineProperty,
   ObjectSetPrototypeOf,
@@ -364,6 +365,11 @@ class Interface extends InterfaceConstructor {
     return Infinity;
   }
 
+  get rows() {
+    if (this.output?.rows) return this.output.rows;
+    return Infinity;
+  }
+
   /**
    * Sets the prompt written to the output.
    * @param {string} prompt
@@ -500,6 +506,10 @@ class Interface extends InterfaceConstructor {
     // cursor position
     const cursorPos = this.getCursorPos();
 
+    const terminalRows = this.rows;
+
+    const exceedsTerminal = lineRows >= terminalRows && terminalRows !== Infinity;
+
     // First move to the bottom of the current line, based on cursor pos
     const prevRows = this.prevRows || 0;
     if (prevRows > 0) {
@@ -513,16 +523,41 @@ class Interface extends InterfaceConstructor {
 
     if (this[kIsMultiline]) {
       const lines = StringPrototypeSplit(this.line, '\n');
-      // Write first line with normal prompt
-      this[kWriteToOutput](this[kPrompt] + lines[0]);
 
-      // For continuation lines, add the "|" prefix
-      for (let i = 1; i < lines.length; i++) {
+      let startLine = 0;
+      let endLine = lines.length;
+      let visibleCursorRow = cursorPos.rows;
+
+      if (exceedsTerminal) {
+        const maxVisibleLines = terminalRows - 2;
+
+        const halfViewport = MathFloor(maxVisibleLines / 2);
+
+        if (cursorPos.rows < halfViewport) {
+          startLine = 0;
+          endLine = MathMin(lines.length, maxVisibleLines);
+        } else if (cursorPos.rows >= lines.length - halfViewport) {
+          startLine = MathMax(0, lines.length - maxVisibleLines);
+          endLine = lines.length;
+          visibleCursorRow = cursorPos.rows - startLine;
+        } else {
+          startLine = cursorPos.rows - halfViewport;
+          endLine = MathMin(lines.length, startLine + maxVisibleLines);
+          visibleCursorRow = halfViewport;
+        }
+      }
+
+      this[kWriteToOutput](this[kPrompt] + lines[startLine]);
+
+      for (let i = startLine + 1; i < endLine; i++) {
         this[kWriteToOutput](`\n${kMultilinePrompt.description}` + lines[i]);
       }
+
+      this.prevRows = exceedsTerminal ? visibleCursorRow : cursorPos.rows;
     } else {
       // Write the prompt and the current buffer content.
       this[kWriteToOutput](line);
+      this.prevRows = cursorPos.rows;
     }
 
     // Force terminal to allocate a new line
@@ -537,8 +572,6 @@ class Interface extends InterfaceConstructor {
     if (diff > 0) {
       moveCursor(this.output, 0, -diff);
     }
-
-    this.prevRows = cursorPos.rows;
   }
 
   /**

test/parallel/test-repl-multiline-navigation.js

@@ -251,3 +251,71 @@ tmpdir.refresh();
     checkResults
   );
 }
+
+{
+  const historyPath = tmpdir.resolve(`.${Math.floor(Math.random() * 10000)}`);
+
+  // Test for issue #59938: REPL cursor navigation in viewport overflow
+  // This test just verifies cursor position consistency when input exceeds
+  // terminal height. The actual screen rendering fix is harder to test
+  // but can be manually verified.
+  class MockOutput extends stream.Writable {
+    constructor() {
+      super({
+        write(chunk, _, next) {
+          next();
+        }
+      });
+      this.columns = 80;
+      this.rows = 20;
+    }
+  }
+
+  const checkResults = common.mustSucceed((r) => {
+    r.write('const arr = [');
+    r.input.run([{ name: 'enter' }]);
+
+    for (let i = 1; i <= 25; i++) {
+      r.write(`  ${i},`);
+      r.input.run([{ name: 'enter' }]);
+    }
+
+    r.write('];');
+
+    const endCursor = r.cursor;
+    assert.ok(endCursor > 0);
+
+    for (let i = 0; i < 27; i++) {
+      r.input.run([{ name: 'up' }]);
+    }
+
+    assert.strictEqual(r.cursor, 0);
+
+    const prevRowsAtTop = r.prevRows;
+    assert.ok(prevRowsAtTop !== undefined);
+
+    for (let i = 0; i < 10; i++) {
+      r.input.run([{ name: 'down' }]);
+    }
+
+    const midCursor = r.cursor;
+    assert.ok(midCursor > 0 && midCursor < endCursor);
+
+    // Navigate back to end
+    for (let i = 0; i < 17; i++) {
+      r.input.run([{ name: 'down' }]);
+    }
+
+    assert.strictEqual(r.cursor, endCursor);
+  });
+
+  repl.createInternalRepl(
+    { NODE_REPL_HISTORY: historyPath },
+    {
+      terminal: true,
+      input: new ActionStream(),
+      output: new MockOutput(),
+    },
+    checkResults
+  );
+}