PageLayout on_touch_down doesn't check for length of self.children. Can get IndexError: list index out of range.

[AJRepo]

Software Versions

• Python: Python 3.11.6
• OS: Debian
• Kivy: 2.2.1-2
• Kivy installation method: dpkg and git

Describe the bug

Using PageLayout with on_touch_down() can generate an IndexError: list index out of range and crash the program using just standard user interactions.

Expected behavior
A clear and concise description of what you expected to happen.

No crash.

To Reproduce
A short, runnable example that reproduces the issue with latest kivy master.

1. Create a simple PageLayout system
2. Add the following code to your main.py

class MyPager(PageLayout):
    """See .kv file for details"""

    def on_touch_down(self, touch): 
        super().on_touch_down(touch)
        if touch.spos[0] > .95:
            if self.page == 1:
                self.page = 0
            else:
                self.page = 1

3. instead of just clicking with a mouse, click-drag the mouse to drag out the page. The page will, instead of stopping a bit before the left side, continue and leave a blank on the right side.
4. Mouse Click one more time
5. Crash

Code and Logs and screenshots

Python File:

rom kivy.app import App
from kivy.lang import Builder
from kivy.uix.pagelayout import PageLayout
from kivy.uix.boxlayout import BoxLayout

KV = """
MyPager:

<MyPager>:
    border: '25dp'
    PageOne:
        Widget:
        Button:
            text: "page1"
        Widget:
    PageTwo:
        Widget:
        Button:
            text: "page2"
        Widget:
"""
class MyPager(PageLayout):
    """See .kv file for details"""

    def on_touch_down(self, touch):
        #Edit:  having this line below was a key part of the the problem. Updating ticket for future readers
        super().on_touch_down(touch)

        print("In main.py:", self)
        print("in main.py page=", self.page)
        print("In main.py: pos=",touch.pos)
        if touch.spos[0] > .95:
            if self.page == 1:
                self.page = 0
            elif self.page == 0:
                self.page = 1
            #Edit:  NOT return True was a key part of the the problem. Updating ticket for future readers
            return True

        #Edit:  NOT having this return was a key part of the the problem. Updating ticket for future readers
        return super().on_touch_down(touch)

class PageOne(BoxLayout):
    """See .kv file for details"""

class PageTwo(BoxLayout):
    """See .kv file for details"""

class TestMyPager(App): #pylint: disable=too-many-instance-attributes
    """docstring"""

    def build(self):
        """docstring"""
        return Builder.load_string(KV)

TestMyPager().run()

Additional context
Add any other context about the problem here.

I have fixed this out of range issue by adding a check on the list length and will submit a pull request shortly.

[AJRepo]

On Startup: the variable page=0, however with click_drag it changes to page=2 even though there are only two pages defined (0 and 1)

Looks like the root of the issue might be in on_touch_move which changes page , but independent of that issue - there should be a range check in on_touch_down which this PR fixes.

[ElliotGarbus]

Edit: The on_touch_down() method needs to be changed to consume the touch when the touch_down is accepted.

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.pagelayout import PageLayout


kv = """
MyPager:

<MyPager>:
    Button:
        text: "p1"
    Button:
        text: "p2"
"""


class MyPager(PageLayout):

    def on_touch_down(self, touch):
        print(f'on_touch_down: {self.page=} {touch.spos[0] > .95=}')
        if touch.spos[0] > .95:
            self.page = 0 if self.page == 1 else 1
            return True
        return super().on_touch_down(touch)


class AnalysisPageLayoutApp(App):
    def build(self):
        return Builder.load_string(kv)


AnalysisPageLayoutApp().run()

[AJRepo]

ok thanks - closing ticket as "not planned"