Different rendering for focused or unfocused ScrollablePane

The following code demonstrates a rendering difference between focused and unfocused mode:

import asyncio
from time import sleep

from prompt_toolkit.application import Application
from prompt_toolkit.buffer import Buffer
from prompt_toolkit.key_binding import KeyBindings
from prompt_toolkit.layout import ScrollablePane, HSplit, Layout, Window
from prompt_toolkit.layout.controls import BufferControl
from prompt_toolkit.widgets import Frame, TextArea


class Runner:
    def __init__(self):
        self.buffer = Buffer()
        self.buffer_control = BufferControl(buffer=self.buffer)
        self.scrollable_pane = ScrollablePane(
            Window(self.buffer_control, wrap_lines=True), show_scrollbar=True, height=5
        )
        self.text_control = TextArea(
            prompt="Your username: ", multiline=False, password=False
        )
        self.focused_pane = Frame(self.text_control)

        root_container = HSplit([self.scrollable_pane, self.focused_pane])
        self.layout = Layout(root_container)

        self.app = Application(layout=self.layout)

    async def run(self):
        asyncio.create_task(self.update_buffer())
        kb = KeyBindings()

        @kb.add("c-q")
        def _(event) -> None:
            event.app.exit()

        self.layout.focus(self.focused_pane)
        await self.app.run_async()

    async def run_pane_focused(self):
        asyncio.create_task(self.update_buffer())
        kb = KeyBindings()

        @kb.add("c-q")
        def _(event) -> None:
            event.app.exit()

        self.layout.focus(self.scrollable_pane)
        await self.app.run_async()

    async def update_buffer(self):
        count = 0
        while count < 10:
            self.buffer.insert_text(f"New Line {count}\n")
            count += 1
            self.scrollable_pane.vertical_scroll = count
            await asyncio.sleep(1)

        self.app.exit()


if __name__ == "__main__":
    runner = Runner()
    # asyncio.run(runner.run())
    asyncio.run(runner.run_pane_focused())

Toggle the different runner methods to see the difference.

Start by running the Python reproduction with ScrollablePane focused and unfocused, then inspect ScrollablePane's rendering and focus handling. Compare both modes while the buffer updates; done means the pane renders consistently in either focus state.