Fix TextArea scrollbar position not updated after paste

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Fixed
 
+- Fixed `TextArea` scrollbar position not updated after paste https://github.com/Textualize/textual/issues/4852
 - Fixed `TextArea` cursor display on wrapped lines https://github.com/Textualize/textual/pull/6196
 - Fixed `remove_children` not refreshing layout https://github.com/Textualize/textual/pull/6206
 - Fixed flicker with :hover pseudo class https://github.com/Textualize/textual/pull/6214

src/textual/widgets/_text_area.py

@@ -1850,6 +1850,7 @@ async def _on_paste(self, event: events.Paste) -> None:
             return
         if result := self._replace_via_keyboard(event.text, *self.selection):
             self.move_cursor(result.end_location)
+            self.scroll_cursor_visible()
             self.focus()
 
     def cell_width_to_column_index(self, cell_width: int, row_index: int) -> int:
@@ -2530,6 +2531,7 @@ def action_paste(self) -> None:
         clipboard = self.app.clipboard
         if result := self._replace_via_keyboard(clipboard, *self.selection):
             self.move_cursor(result.end_location)
+            self.scroll_cursor_visible()
 
     def action_delete_to_start_of_line(self) -> None:
         """Deletes from the cursor location to the start of the line."""

tests/text_area/test_textarea_cut_copy_paste.py

@@ -50,3 +50,116 @@ async def test_paste():
         assert text_area.text == "Hello, World"
         await pilot.press("ctrl+v")
         assert text_area.text == "Hello, WorldWorld"
+
+
+async def test_paste_scrollbar_position_updated():
+    """Regression test for issue #4852: scrollbar position should be updated after paste.
+
+    When pasting text that extends beyond the visible area, the scrollbar should
+    automatically scroll to show the cursor position after the pasted text.
+    """
+    app = TextAreaApp()
+    async with app.run_test(size=(40, 10)) as pilot:
+        text_area = app.query_one(TextArea)
+        await pilot.click(text_area)
+
+        # Create long text that will require scrolling
+        long_text = "\n".join(f"Line {i}" for i in range(50))
+        app.copy_to_clipboard(long_text)
+
+        # Get initial scroll position
+        initial_scroll_x, initial_scroll_y = text_area.scroll_offset
+
+        # Paste the text
+        await pilot.press("ctrl+v")
+
+        # Wait for the paste to complete and scrollbar to update
+        await pilot.pause()
+
+        # Get scroll position after paste
+        final_scroll_x, final_scroll_y = text_area.scroll_offset
+
+        # The scrollbar should have scrolled down to show the cursor
+        # (which should be at the end of the pasted text)
+        assert final_scroll_y > initial_scroll_y, (
+            f"Scrollbar should have scrolled down after paste. "
+            f"Initial: {initial_scroll_y}, Final: {final_scroll_y}"
+        )
+
+        # Verify the text was pasted
+        assert text_area.text == long_text
+
+
+async def test_paste_event_scrollbar_position_updated():
+    """Regression test for issue #4852: scrollbar position should be updated after paste event.
+
+    Tests the _on_paste method specifically (paste via event, not action).
+    """
+    from textual.events import Paste
+
+    app = TextAreaApp()
+    async with app.run_test(size=(40, 10)) as pilot:
+        text_area = app.query_one(TextArea)
+        await pilot.click(text_area)
+
+        # Create long text that will require scrolling
+        long_text = "\n".join(f"Line {i}" for i in range(50))
+
+        # Get initial scroll position
+        initial_scroll_x, initial_scroll_y = text_area.scroll_offset
+
+        # Post a paste event
+        app.post_message(Paste(long_text))
+        await pilot.pause()
+
+        # Get scroll position after paste
+        final_scroll_x, final_scroll_y = text_area.scroll_offset
+
+        # The scrollbar should have scrolled down to show the cursor
+        assert final_scroll_y > initial_scroll_y, (
+            f"Scrollbar should have scrolled down after paste event. "
+            f"Initial: {initial_scroll_y}, Final: {final_scroll_y}"
+        )
+
+        # Verify the text was pasted
+        assert text_area.text == long_text
+
+
+async def test_paste_scrollbar_with_wide_text():
+    """Regression test for issue #4852: scrollbar position updated with wide text.
+
+    Based on comment from TomJGooding: vertical scrollbar updates correctly
+    when horizontal scrollbar is also triggered. This test verifies both scrollbars work.
+    """
+    app = TextAreaApp()
+    async with app.run_test(size=(40, 10)) as pilot:
+        text_area = app.query_one(TextArea)
+        await pilot.click(text_area)
+
+        # Create text that requires both horizontal and vertical scrolling
+        # Wide lines + many lines
+        wide_text = "\n".join(f"Line {i}: {'x' * 100}" for i in range(50))
+        app.copy_to_clipboard(wide_text)
+
+        # Get initial scroll position
+        initial_scroll_x, initial_scroll_y = text_area.scroll_offset
+
+        # Paste the text
+        await pilot.press("ctrl+v")
+        await pilot.pause()
+
+        # Get scroll position after paste
+        final_scroll_x, final_scroll_y = text_area.scroll_offset
+
+        # Both scrollbars should have updated
+        # Vertical scrollbar should have scrolled down
+        assert final_scroll_y > initial_scroll_y, (
+            f"Vertical scrollbar should have scrolled down after paste. "
+            f"Initial: {initial_scroll_y}, Final: {final_scroll_y}"
+        )
+
+        # Horizontal scrollbar may or may not scroll depending on cursor position
+        # But the important thing is that vertical scrollbar updated correctly
+
+        # Verify the text was pasted
+        assert text_area.text == wide_text