TextTruncateEllipsis abnormally truncates strings with multi-byte UTF-8 characters

[glenvan]

Checklist

• I have searched the issue tracker for open issues that relate to the same problem, before opening a new one.
• This issue only relates to a single bug. I will open new issues for any other problems.

Describe the bug

I found something odd with widget.Label when using .Truncate = fyne.TextTruncateEllipsis and multi-byte UTF-8 characters: a number of trailing characters are missing or not rendered according to the number of bytes in the multi-byte character.

For example, a two-byte chacter like 'Š' will trim 1 character from the end of the string, a three-byte character like '∞' will trim two characters, and a four-byte character like '🪃' will trim three characters from the end of the string.

Note that I'm not talking about long labels that need to be truncated based on available screen size – I'm talking about any label. It just needs to have multi-byte characters present and be using fyne.TextTruncatedEllipsis.

How to reproduce

1. label := widget.NewLabel("∞ is more than 12345") or use your favorite multi-byte UTF-8 character
2. label.Truncation = fyne.TextTruncateEllipsis
3. Add the label to your layout and run

Screenshots

Example code in action:

Example code

package main

import (
	"fyne.io/fyne/v2"
	"fyne.io/fyne/v2/app"
	"fyne.io/fyne/v2/container"
	"fyne.io/fyne/v2/widget"
)

func main() {
	a := app.New()

	w := a.NewWindow("UTF-8 Multi-byte Ellipsis Truncation")
	w.SetMaster()

	labelTruncateOff := widget.NewLabel("TextTruncateOff")
	labelTruncateOff.TextStyle.Bold = true
	labelTruncateClip := widget.NewLabel("TextTruncateClip")
	labelTruncateClip.TextStyle.Bold = true
	labelTruncateEllipsis := widget.NewLabel("TextTruncateEllipsis")
	labelTruncateEllipsis.TextStyle.Bold = true
	labelTruncateEllipsis.TextStyle.Italic = true

	label1Byte := widget.NewLabel("1-byte")
	label1Byte.TextStyle.Bold = true
	label2Byte := widget.NewLabel("2-byte")
	label2Byte.TextStyle.Bold = true
	label3Byte := widget.NewLabel("3-byte")
	label3Byte.TextStyle.Bold = true
	label4Byte := widget.NewLabel("4-byte")
	label4Byte.TextStyle.Bold = true

	// Note: Monospace is used here because it seems to support more of unicode than the bundled
	// Noto fonts, it's not critical to reproduce the issue

	testLabel1 := widget.NewLabel("x 0123456789")
	testLabel1.TextStyle = fyne.TextStyle{Monospace: true}
	testLabel1.Truncation = fyne.TextTruncateOff

	testLabel2 := widget.NewLabel("Š 0123456789")
	testLabel2.TextStyle = fyne.TextStyle{Monospace: true}
	testLabel2.Truncation = fyne.TextTruncateOff

	testLabel3 := widget.NewLabel("∞ 0123456789")
	testLabel3.TextStyle = fyne.TextStyle{Monospace: true}
	testLabel3.Truncation = fyne.TextTruncateOff

	testLabel4 := widget.NewLabel("🪃 0123456789")
	testLabel4.TextStyle = fyne.TextStyle{Monospace: true}
	testLabel4.Truncation = fyne.TextTruncateOff

	testLabel5 := widget.NewLabel("x 0123456789")
	testLabel5.TextStyle = fyne.TextStyle{Monospace: true}
	testLabel5.Truncation = fyne.TextTruncateClip

	testLabel6 := widget.NewLabel("Š 0123456789")
	testLabel6.TextStyle = fyne.TextStyle{Monospace: true}
	testLabel6.Truncation = fyne.TextTruncateClip

	testLabel7 := widget.NewLabel("∞ 0123456789")
	testLabel7.TextStyle = fyne.TextStyle{Monospace: true}
	testLabel7.Truncation = fyne.TextTruncateClip

	testLabel8 := widget.NewLabel("🪃 0123456789")
	testLabel8.TextStyle = fyne.TextStyle{Monospace: true}
	testLabel8.Truncation = fyne.TextTruncateClip

	testLabel9 := widget.NewLabel("x 0123456789")
	testLabel9.TextStyle = fyne.TextStyle{Monospace: true}
	testLabel9.Truncation = fyne.TextTruncateEllipsis

	testLabel10 := widget.NewLabel("Š 0123456789")
	testLabel10.TextStyle = fyne.TextStyle{Monospace: true}
	testLabel10.Truncation = fyne.TextTruncateEllipsis

	testLabel11 := widget.NewLabel("∞ 0123456789")
	testLabel11.TextStyle = fyne.TextStyle{Monospace: true}
	testLabel11.Truncation = fyne.TextTruncateEllipsis

	testLabel12 := widget.NewLabel("🪃 0123456789")
	testLabel12.TextStyle = fyne.TextStyle{Monospace: true}
	testLabel12.Truncation = fyne.TextTruncateEllipsis

	w.SetContent(container.NewStack(container.NewGridWithColumns(
		4,
		widget.NewLabel(" "),
		labelTruncateOff,
		labelTruncateClip,
		labelTruncateEllipsis,
		label1Byte,
		testLabel1,
		testLabel5,
		testLabel9,
		label2Byte,
		testLabel2,
		testLabel6,
		testLabel10,
		label3Byte,
		testLabel3,
		testLabel7,
		testLabel11,
		label4Byte,
		testLabel4,
		testLabel8,
		testLabel12,
	)))

	w.ShowAndRun()
}

Fyne version

2.4.0

Go compiler version

1.21.1

Operating system and version

Windows 10, macOS Ventura 13.6

Additional Information

No response