Weird behaviour of canvas.Line.Move()

[pwiecz]

Describe the bug:

Calling Move(pos) on canvas.Line can make the line rotate.
In the implementation of Move(pos) Size() is used to preserve relative positions of Position1 and Position2, but Size() calls math.Abs on the difference of coordinates, so it does not really represent correct relative positions of Position1 and Position2.

To Reproduce:

Steps to reproduce the behaviour:

1. Create a widget with a line, where line.Position1.x > line.Position2.x or line.Position1.y > line.Position2.y
2. Call Move() on the line in response to some event
3. Call Refresh() on the line to rasterize it again
4. See that the line flips its orientation

Example code:

package main

import (
	"image/color"

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

type testWidget struct {
	widget.BaseWidget
	background     *canvas.Rectangle
	lineX0, lineY0 float32
	line           *canvas.Line
	objects        []fyne.CanvasObject
}

func NewTestWidget() *testWidget {
	w := &testWidget{}
	w.ExtendBaseWidget(w)
	w.background = canvas.NewRectangle(color.White)
	w.line = canvas.NewLine(color.NRGBA{0, 0, 255, 255})
	w.lineX0 = 10
	w.lineY0 = 10
	w.line.Position1 = fyne.Position{10, 50}
	w.line.Position2 = fyne.Position{50, 10}
	w.objects = []fyne.CanvasObject{w.background, w.line}
	return w
}
func (w *testWidget) CreateRenderer() fyne.WidgetRenderer { return w }
func (w *testWidget) Layout(size fyne.Size)               { w.background.Resize(size) }
func (w *testWidget) MinSize() fyne.Size                  { return fyne.Size{640, 480} }
func (w *testWidget) Refresh()                            {}
func (w *testWidget) Objects() []fyne.CanvasObject        { return w.objects }
func (w *testWidget) Destroy()                            {}
func (w *testWidget) Dragged(event *fyne.DragEvent) {
	dx, dy := event.Dragged.DX, event.Dragged.DY
	w.lineX0 += dx
	w.lineY0 += dy
	w.line.Move(fyne.Position{w.lineX0, w.lineY0})
	w.line.Refresh()
}
func (w *testWidget) DragEnd() {}

func main() {
	a := app.New()
	w := a.NewWindow("Test line")
	w.SetContent(NewTestWidget())
	w.ShowAndRun()
}

Device (please complete the following information):

• OS: Linux
• Version: 5.4.10-72 Ubuntu
• Go version: 1.16.2
• Fyne version: 2.0.2

[andydotxyz]

Thanks for the report, #2240 should fix it :)