Get Slider range to change with Manipulate variable?

Questions
1

All:

I’m working on a demonstration, and having an issue getting the slider values to cooperate. The code below isolates the issue. I’d like for the a value to reset to the new initial value when I click the button. In this code, the slider boundaries change (which is good), but the existing a value is inherited rather than resetting to the new initial value.

Appreciate any and all advice!

Manipulate[
 
 a,
 
 {{b, 1, "direction"}, {1 -> "Left to Right", 2 -> "Right to Left"}},
 PaneSelector[{
   1 -> Control[{{a, -1, "x"}, -1, 1.99999, Appearance -> "Labeled"}],
   2 -> Control[{{a, 4, "x"}, 2.00001, 4, Appearance -> "Labeled"}]
   }, Dynamic[b]],
 
 TrackedSymbols -> {a, b}
 ]
2

Maybe something similar to this?

Manipulate[
 	If[dir != oldDir, a = -a; oldDir = dir];
 	a,
 	{{dir, 1, "direction"}, {1 -> "Positive", 2 -> "Negative"}},
 	Dynamic@Switch[dir,
   		1, Control@{{a, 0}, 0, 2, Appearance -> "Labeled"},
   		2, Control@{{a, 0}, -2, 0, Appearance -> "Labeled"}
   	],
 	{oldDir, 1, ControlType -> None},
 	TrackedSymbols :> {dir, a}, AppearanceElements -> None
 ]