widget: reset Editor x offset on mouse initiated caret movement

The caret x-offset tracks residual horizontal offset for arrow key
movements. Caret movement by the mouse should reset the residual.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

widget/editor.go

@@ -441,6 +441,7 @@ func (e *Editor) moveCoord(pos image.Point) {
 		carLine++
 	}
 	x := fixed.I(pos.X + e.scrollOff.X)
+	e.carXOff = 0
 	e.moveToLine(x, carLine)
 }