Scrollbars may trap the mouse if hidden while the user is scrolling

[xavierog]

Current behaviour

If a scrollable widget is programmatically hidden while the user is scrolling by dragging the scrollbar handle, all mouse events are seemingly blocked (they are actually trapped by the invisible scrollbar).

This MRE reproduces the issue by programmatically collapsing a Collapsible that contains a scrollable RichLog:

#!/usr/bin/env python3
from textual.app import App, ComposeResult
from textual.widgets import Label, Collapsible, RichLog

INSTRUCTIONS = """[u]Instructions:[/u]
1. Using your mouse, grab the scrollbar handle of the currently focused Collapsible
2. Move it up and down -- [b]do NOT release the mouse button[/b]
3. A few seconds later, the timer should collapse the Collapsible
4. Release the mouse button
5. Try clicking somewhere else, e.g. on the second collapsible
[u]Result:[/u] the mouse is "trapped" by the hidden scrollbar.
[i]Bonus:[/i] Using Tab and Enter, expand the Collapsible and move your mouse.
"""
FILLER = 'Lorem ipsum dolor sit amet, consectetur adipiscing elit\n'
COLLAPSIBLE_TITLE='Collapse me by pressing Enter while moving the scrollbar with your mouse!'

class MouseTrap(App):
	def compose(self) -> ComposeResult:
		yield Label(INSTRUCTIONS)
		for _ in range(2):
			with Collapsible(title=COLLAPSIBLE_TITLE, collapsed=False):
				richlog = RichLog()
				richlog.styles.height = 15
				richlog.write(FILLER * 150)
				yield richlog

	def on_ready(self) -> None:
		self.set_timer(7, self.collapse)

	def collapse(self) -> None:
		self.query('Collapsible').first(Collapsible).collapsed = True

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

Expectations

The scrollbar handle should be "released" as soon as the scrollbar is hidden so that mouse events resume their normal course.

Textual Diagnostics

Versions

Name	Value
Textual	0.52.1 / commit f55610e
Rich	13.7.0

Python

Name	Value
Version	3.11.8
Implementation	CPython
Compiler	GCC 13.2.0
Executable	~/whatever/venv/bin/python3

Operating System

Name	Value
System	Linux
Release	6.6.15-amd64
Version	#1 SMP PREEMPT_DYNAMIC Debian 6.6.15-2 (2024-02-04)

Terminal

Name	Value
Terminal Application	tmux (3.4)
TERM	tmux-256color
COLORTERM	truecolor
FORCE_COLOR	Not set
NO_COLOR	Not set

Rich Console options

Name	Value
size	width=283, height=58
legacy_windows	False
min_width	1
max_width	283
is_terminal	False
encoding	utf-8
max_height	58
justify	None
overflow	None
no_wrap	False
highlight	None
markup	None
height	None

[willmcgugan]

At a guess, I suspect it is the mouse capture system not "releasing" the mouse, if the scrollbar is made invisible.

Feels like an easy fix.

[TomJGooding]

I'm sure the maintainers are way ahead of me on this, but posting my findings anyway in case it helps...

The ScrollBar does already release the mouse on a Hide event, but perhaps related is #3460 where what Hide means exactly needs some clarification.

textual/src/textual/scrollbar.py

Lines 291 to 294 in c7370f3

	def _on_hide(self, event: events.Hide) -> None:
	if self.grabbed:
	self.release_mouse()
	self.grabbed = None

But more importantly this issue isn't only with scrollbars, but any widget which captures the mouse. For example, if you start a mouse selection with the TextArea which is then hidden, you'll see the same behaviour.

[davep]

Just to confirm things: I think @TomJGooding has perfectly summed up the issue here. The situation presented with the MRE above won't result in a Hide event being posted, which does feed back into #3460.

[davep]

Digging deeper, what is interesting is the other contents do get a Hide message, which further suggests that the issue here isn't that Hide isn't getting posted, it's that a scrollbar isn't taken into account when sending such messages. For example, the problem documented here is fixed with:

diff --git a/src/textual/screen.py b/src/textual/screen.py
index f9674a47..1cee5b11 100644
--- a/src/textual/screen.py
+++ b/src/textual/screen.py
@@ -793,6 +793,8 @@ class Screen(Generic[ScreenResultType], Widget):
 
                 for widget in hidden:
                     widget.post_message(Hide())
+                    if widget.vertical_scrollbar is not None:
+                        widget.vertical_scrollbar.post_message(Hide())
 
                 # We want to send a resize event to widgets that were just added or change since last layout
                 send_resize = shown | resized

while likely not the actual fix to go with, I think it does suggest that it isn't down to a specific event not being posted, it's just that it isn't being posted as widely as it should.

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[xavierog]

Now that the fix is available in main, I confirm the issue no longer happens on my side (tested with both the MRE and my application). Thanks for your work.