Wrapped text with a TTF font is rendered poorly

[igorhub]

Hi, everyone! I noticed that when I use a TTF font and wrap text in a rich text widget (or even a simple label) multiple times, the quality of rendering degrades.

When the text is wrapped once (or not wrapped at all), it looks sharp:

When it's wrapped twice or more, parts of it are blurry:

Can anyone shed some light on this? I’m on Linux, using fyne v2.4.1. I haven’t found anything in issues about the matter. Not filing an issue yet, since I have very little knowledge of fonts, rendering, anti-aliasing, all that stuff, and this may likely be a mistake on my part.

My code:

package fynepg

import (
	"image/color"

	"fyne.io/fyne/v2"
	"fyne.io/fyne/v2/app"
	"fyne.io/fyne/v2/theme"
	"fyne.io/fyne/v2/widget"
)

type sourceSerifTheme struct {
	font fyne.Resource
}

func newSourceSerifTheme() *sourceSerifTheme {
	path := "/storage/fonts/source-serif/source-serif-4.004/TTF/SourceSerif4-Regular.ttf"
	r, err := fyne.LoadResourceFromPath(path)
	if err != nil {
		panic(err)
	}
	return &sourceSerifTheme{font: r}
}

func (*sourceSerifTheme) Color(name fyne.ThemeColorName, v fyne.ThemeVariant) color.Color {
	return theme.DefaultTheme().Color(name, v)
}

func (t *sourceSerifTheme) Font(style fyne.TextStyle) fyne.Resource {
	return t.font
}

func (*sourceSerifTheme) Icon(name fyne.ThemeIconName) fyne.Resource {
	return theme.DefaultTheme().Icon(name)
}

func (*sourceSerifTheme) Size(name fyne.ThemeSizeName) float32 {
	return theme.DefaultTheme().Size(name)
}

func runBadRendering() {
	a := app.New()
	w := a.NewWindow("fynetest")
	label := widget.NewLabel("Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Nisi est sit amet facilisis magna etiam. Blandit libero volutpat sed cras ornare arcu dui vivamus.")
	label.Wrapping = fyne.TextWrapWord
	w.SetContent(label)
	a.Settings().SetTheme(newSourceSerifTheme())
	w.ShowAndRun()
}

[igorhub]

In fact, this happens even with the stock font -- it is just less apparent. It takes some time, but after resizing the window back and forth, you can get output like this:

(see how the last line looks compared to the others)

[igorhub]

Filed an issue: #4390