List widget panic when refreshing after changing content through combobox changed.

[changsheng1239]

Describe the bug:

If I display the list with 20 items but did not scroll it at all, changed the value of list and refresh. the list will display 1 item successfully. Whereas if I display 20 items and scrolled down, change the value of list and refresh will panic immediately with that error stack.

To Reproduce:

Steps to reproduce the behaviour:

1. Run the example code.
2. Change combo box value to 20.
3. Scroll down for a bit.
4. Change combo box value to 1.
5. panic.

If step 3 is ignored, it will run successfully.

Example code:

package main

import (
	"strconv"

	"fyne.io/fyne/v2"
	"fyne.io/fyne/v2/app"
	"fyne.io/fyne/v2/container"
	"fyne.io/fyne/v2/layout"
	"fyne.io/fyne/v2/widget"
)

func main() {
	myApp := app.New()
	myWindow := myApp.NewWindow("List example")

	userTab, _ := renderUsersTab(myWindow)

	tabs := container.NewAppTabs(
		container.NewTabItem("Users", userTab),
	)
	tabs.SetTabLocation(container.TabLocationTop)

	myWindow.SetContent(tabs)
	myWindow.CenterOnScreen()
	myWindow.Resize(fyne.Size{Width: 700, Height: 500})
	myWindow.ShowAndRun()
}

func renderUsersTab(w fyne.Window) (fyne.CanvasObject, error) {
	groups := []string{"1", "10", "20"}
	members := generateEmails(1)

	l := widget.NewList(func() int {
		return len(members)
	}, func() fyne.CanvasObject {
		return widget.NewLabel("PLACEHOLDERS@DOMAIN.COM")
	}, func(lii widget.ListItemID, co fyne.CanvasObject) {
		if lii >= len(members) {
			return
		}
		val := members[lii]
		label := co.(*widget.Label)
		label.SetText(val)
	})

	groupLabel := widget.NewLabel("Groups")
	groupCombo := widget.NewSelect(groups, func(value string) {
		num, _ := strconv.Atoi(value)
		members = generateEmails(num)
		l.Refresh()
	})

	grid := container.New(layout.NewFormLayout(), groupLabel, groupCombo)
	tab := container.New(layout.NewBorderLayout(grid, nil, nil, nil), grid, l)
	return tab, nil
}

func generateEmails(num int) []string {
	emails := []string{}

	for i := 0; i < num; i++ {
		emails = append(emails, "testuser"+strconv.Itoa(i)+"@gmail.com")
	}

	return emails
}

Device:

• OS: Windows
• Version: 10 Pro
• Go version: go version go1.15.7 windows/amd64
• Fyne version: 2.0.0

[changsheng1239]

If I used widget.Entry instead of Select, it worked as expected.

package main

import (
	"strconv"

	"fyne.io/fyne/v2"
	"fyne.io/fyne/v2/app"
	"fyne.io/fyne/v2/container"
	"fyne.io/fyne/v2/layout"
	"fyne.io/fyne/v2/widget"
)

func main() {
	myApp := app.New()
	myWindow := myApp.NewWindow("List example")

	userTab, _ := renderUsersTab(myWindow)

	tabs := container.NewAppTabs(
		container.NewTabItem("Users", userTab),
	)
	tabs.SetTabLocation(container.TabLocationTop)

	myWindow.SetContent(tabs)
	myWindow.CenterOnScreen()
	myWindow.Resize(fyne.Size{Width: 700, Height: 500})
	myWindow.ShowAndRun()
}

func renderUsersTab(w fyne.Window) (fyne.CanvasObject, error) {
	members := generateEmails(1)

	l := widget.NewList(func() int {
		return len(members)
	}, func() fyne.CanvasObject {
		return widget.NewLabel("PLACEHOLDERS@DOMAIN.COM")
	}, func(lii widget.ListItemID, co fyne.CanvasObject) {
		if lii >= len(members) {
			return
		}
		val := members[lii]
		label := co.(*widget.Label)
		label.SetText(val)
	})

	groupLabel := widget.NewLabel("Groups")
	groupEntry := widget.NewEntry()
	groupEntry.OnChanged = func(s string) {
		num, _ := strconv.Atoi(s)
		members = generateEmails(num)
		l.Refresh()
	}

	grid := container.New(layout.NewFormLayout(), groupLabel, groupEntry)
	tab := container.New(layout.NewBorderLayout(grid, nil, nil, nil), grid, l)
	return tab, nil
}

func generateEmails(num int) []string {
	emails := []string{}

	for i := 0; i < num; i++ {
		emails = append(emails, "testuser"+strconv.Itoa(i)+"@gmail.com")
	}

	return emails
}

[andydotxyz]

Fix on develop and release/v2.0.x for testing