Giving focus to a widget not scrolled into view doesn't always scroll it into view

[davep]

Using the following code:

from textual.app import App, ComposeResult
from textual.containers import VerticalScroll
from textual.widgets import Label, Input

class Inner(Label, can_focus=True):
    DEFAULT_CSS = """
    Inner {
        width: 1fr;
        border: solid grey;
        &:focus {
            border: thick green;
        }
    }
    """

class Top(VerticalScroll, can_focus=False):

    def compose(self) -> ComposeResult:
        for n in range(100):
            yield Inner(f"{n}. This is a label that can get focus")

class ScrollFocusIssueApp(App[None]):

    AUTO_FOCUS = "Input"

    def compose(self) -> ComposeResult:
        yield Top()
        yield Input()

    def on_mount(self) -> None:
        self.query_one(Top).scroll_end(animate=False)

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

often, but not always, if I tab or shift+tab from the Input to the Inner that isn't currently visible in the Top widget, the focused widget isn't scrolled into view. A subsequent change of focus "further" in the same direction will then result in the subsequently-focused widget being brought into view.

So, using the above, I can often cause the problem like this:

1. Run up the application
2. Press tab
3. Sometimes, but not always, the focused widget doesn't scroll into view.
4. If it did scroll into view...
5. shift+tab twice
6. For me, without fail, the focused widget isn't scrolled into view

[darrenburns]

I'm pretty sure this is something to do with the animation that occurs when you focus_next. If you switch off animation where scroll_to_center is called inside the focus logic in screen.py, things seem to work consistently.

[darrenburns]

I can't reproduce the issue in Elia though in a chat with only a couple of messages - I'd have expected it to exhibit the same behaviour given this MRE - I wonder if it's related to the distance that needs to be scrolled or the number of children.

[TomJGooding]

I've not dived too deep into this yet, but if you add a debug log here:

diff --git a/src/textual/screen.py b/src/textual/screen.py
index b84fbc277..6dd3d0a43 100644
--- a/src/textual/screen.py
+++ b/src/textual/screen.py
@@ -632,6 +632,7 @@ class Screen(Generic[ScreenResultType], Widget):
 
                     def scroll_to_center(widget: Widget) -> None:
                         """Scroll to center (after a refresh)."""
+                        self.log.debug(f"{widget.has_focus = }")
                         if widget.has_focus and not self.screen.can_view(widget):
                             self.screen.scroll_to_center(widget, origin_visible=True)

It looks like the scroll_to_center isn't being called because widget.has_focus is False.

[darrenburns]

Weird, because I added an "animate=False" inside that same block and it seemed to fix the issue. At least, I think it was in there.

[TomJGooding]

@darrenburns I originally tried the same thing but just happened to reproduce the same issue first try, I think the problem is that the bug is very intermittent. The actual issue seems to be when the Focus message is handled to set has_focus (before or after this scroll_to_center function), at least based on my quick look into this.

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