Crash on first `Input` in compose of `app.DEFAULT_MODE` screen

[cognifloyd]

• Have you checked closed issues? (https://github.com/Textualize/textual/issues?q=is%3Aissue+is%3Aclosed)

• Have you checked against the most recent version of Textual? (https://pypi.org/search/?q=textual)

The bug

Initially reported on discord: https://discord.com/channels/1026214085173461072/1033754296224841768/1483472126802661407

I ran into an issue when I tried to add the first Input.

It seems Input cannot be used with app.MODES, or at least not in the compose method of the first screen of the app.DEFAULT_MODE (I did not set app.SCREENS). That kills the app with a traceback for this error:

ScreenStackError: No screens on stack

The crash happens because Inputs watchers end up trying to do self.app.clear_selection().

textual/src/textual/widgets/_input.py

Lines 507 to 508 in 0f0849f

	def _watch_selection(self, selection: Selection) -> None:
	self.app.clear_selection()

That has this try/except:

textual/src/textual/app.py

Lines 4555 to 4560 in 0f0849f

	def clear_selection(self) -> None:
	"""Clear text selection on the active screen."""
	try:
	self.screen.clear_selection()
	except NoScreen:
	pass

That only catches NoScreen, but with modes a ScreenStackError is raised because that first screen hasn't been mounted yet, so the stack is still empty.

For now, I'm working around the issue by adding a clear_selection method to my app that does:

def clear_selection(self):
    try:
        super().clear_selection()
    except ScreenStackError:
        pass

This workaround is based on several places in the code base that do except NoScreen, ScreenStackError, so it seemed like a reasonable fix.

On discord, David said the following:
https://discord.com/channels/1026214085173461072/1033754296224841768/1483494995523534999

Interesting. I can reproduce this using the modes01.py example from the Guide (and replacing the dashboard Placeholder by an Input). HOWEVER, if you do not use DEFAULT_MODE but use the code in the modes01.py example, you don't run into this problem. So do this:

    # DEFAULT_MODE = "dashboard"

    def on_mount(self) -> None:
        self.switch_mode("dashboard")

(change the name of course) and skip using DEFAULT_MODE, at least for now.

Textual Diagnostics

Versions

Name	Value
Textual	8.1.1
Rich	14.3.2

Python

Name	Value
Version	3.13.7
Implementation	CPython
Compiler	Clang 20.1.4
Executable	/home/jafloyd/g/github/copart-pe/ghe.git/.venv/bin/python3

Operating System

Name	Value
System	Linux
Release	6.8.0-90-generic
Version	#91~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Thu Nov 20 15:20:45 UTC 2

Terminal

Name	Value
Terminal Application	Konsole 220403
TERM	xterm-256color
COLORTERM	truecolor
FORCE_COLOR	Not set
NO_COLOR	Not set

Rich Console options

Name	Value
size	width=236, height=55
legacy_windows	False
min_width	1
max_width	236
is_terminal	True
encoding	utf-8
max_height	55
justify	None
overflow	None
no_wrap	False
highlight	None
markup	None
height	None

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