Log widgets do not anchor as expected

[tim-elmer]

Summary

When auto_scroll=False and anchor() are set on a Log or RichLog, the scrolling behavior is not as expected.

Expected Behavior

Based on the documentation for widget, the widget should anchor to the end of its contents unless the user has scrolled away.

I would assume that the widget would re-anchor itself if the user scrolls back to the end of the contents, but this is not described in the documentation.

Actual Behavior

The widget is anchored to the beginning of its contents. If the user scrolls away, the widget respects the scroll as expected. When the user scrolls back to the end of the contents, the widget again anchors to the beginning of its contents.

Screencast_20251025_151020.webm

Example

from datetime import datetime
from time import sleep

from textual import work
from textual.app import App
from textual.widgets import Log


class MyApp(App):
    def compose(self):
        yield Log(auto_scroll=False)

    def on_mount(self):
        self.log_widget = self.query_one(Log)
        self.log_widget.anchor()
        self.write_lines()

    @work(thread=True)
    def write_lines(self):
        while True:
            self.log_widget.write(str(datetime.now()) + "\n")
            sleep(1)

if __name__ == "__main__":
    MyApp().run()

Textual Diagnostics

Versions

Name	Value
Textual	6.4.0
Rich	14.2.0

Python

Name	Value
Version	3.13.9
Implementation	CPython
Compiler	GCC 15.2.1 20250808 (Red Hat 15.2.1-1)
Executable	/home/tim/.local/share/hatch/env/virtual/sweeper/91pWT2mp/sweeper/bin/python

Operating System

Name	Value
System	Linux
Release	6.17.4-200.fc42.x86_64
Version	#1 SMP PREEMPT_DYNAMIC Sun Oct 19 18:47:49 UTC 2025

Terminal

Name	Value
Terminal Application	Konsole 250802
TERM	xterm-256color
COLORTERM	truecolor
FORCE_COLOR	Not set
NO_COLOR	Not set

Rich Console options

Name	Value
size	width=126, height=22
legacy_windows	False
min_width	1
max_width	126
is_terminal	True
encoding	utf-8
max_height	22
justify	None
overflow	None
no_wrap	False
highlight	None
markup	None
height	None

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[tim-elmer]

I was able to work around this with the following hack:

class UnlockingRichLog(RichLog):
    def _scroll_to(
            self,
            x: float | None = None,
            y: float | None = None,
            **kwargs
    ):
        # Unlock if scrolling up
        if isinstance(y, float) and y < self.scroll_y:
            self.auto_scroll = False

        # Lock if scrolling to the end. Uses a fuzzy offset instead of
        #   is_vertical_scroll_end because the exact scroll required when
        #   combined with scrolling output feels bad.
        if (
                isinstance(y, float) and
                y > self.scroll_y and
                y + 3 >= self.max_scroll_y
        ):
            self.auto_scroll = True

        # Actually scroll
        super()._scroll_to(x, y, **kwargs)

[TomJGooding]

I'm not saying this isn't a bug and perhaps I've misunderstood, but isn't this already the default Log behaviour without the need for the anchor?

Here's a quick example (avoiding using a thread worker), which seems to stay scrolled the bottom when new content is added, until the user moves the scroll position?

from datetime import datetime

from textual.app import App, ComposeResult
from textual.widgets import Log


class ExampleApp(App):
    CSS = """
    Log { 
        width: 30;
        height: 10;
    }
    """

    def compose(self) -> ComposeResult:
        yield Log()

    def on_mount(self) -> None:
        self.set_interval(1.0, self.write_line)

    def write_line(self) -> None:
        log = self.query_one(Log)
        log.write_line(str(datetime.now()))


if __name__ == "__main__":
    app = ExampleApp()
    app.run()

[tim-elmer]

Thanks Tom, I think I saw the auto_scroll parameter and didn't bother trying without it. The documentation doesn't describe this behavior, at least that I can tell.

Additionally, the default auto_scroll parameter for RichLog (which is what I'm actually trying to use) is True, and when set to False, it does not exhibit the same behavior as Log; it remains at the initial or last user-scrolled position.

[willmcgugan]

There are two things going on here that need to be unpicked.

First point is that you shouldn't attempt to update the UI directory from a thread, including a threaded worker. Use call_from_thread or send a message.

Second point is that anchor applies to compound widgets only, not line api widgets like Log, which must implement that behavior themselves. The docs don't mention this and of course they should.

Closing this issue, to avoid confusion. Feel free to open another issue or PR if you want the anchoring behavor in Log.

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• Will McGugan