FlxSpriteGroup.set_alpha borked when setting alpha from 0 to 1

[IBwWG]

• Flixel version: 4.0.0
• OpenFL version: 3.6.1
• Lime version: 2.9.1
• Affected targets:
  windows and flash

  ---

Code snippet reproducing the issue:

package;
import flixel.FlxSprite;
import flixel.FlxState;
import flixel.group.FlxSpriteGroup;

class PlayState extends FlxState
{
    override public function create():Void 
    {
        var g:FlxSpriteGroup = cast add(new FlxSpriteGroup());
        g.add(cast new FlxSprite());
        g.members[0].makeGraphic(100, 100);
        g.alpha = 0;
        g.alpha = 1;
    }
}

---

Observed behavior:
Blank.
Expected behavior:
Should see white square, as you would if you commented the last two lines out.

[IBwWG]

This is likely caused by this line in FlxSpriteGroup.hx:

var factor:Float = (alpha > 0) ? Value / alpha : 0;

I have two questions about this:

a) what's the purpose of it? (@Gama11 apparently it's from you, can you help me understand the intention?)
b) in the above example, how come it doesn't cause a divide by zero error? instead it passes 0 on as the value of factor to the next function, which then sets the alpha to 0, causing the above broken behaviour

[Gama11]

Duplicate of #1353.

[IBwWG]

Sorry! I searched before the last bug, forgot to search this time. With that title I definitely would've found it! Whoops!