Feat: allow show() to take optional detailsPromise

 * closes #645

Author: marcoscaceres

index.html

@@ -499,7 +499,7 @@ 
         [Constructor(sequence<PaymentMethodData> methodData, PaymentDetailsInit details, optional PaymentOptions options),
         SecureContext, Exposed=Window]
         interface PaymentRequest : EventTarget {
-          Promise<PaymentResponse> show();
+          Promise<PaymentResponse> show(optional Promise<PaymentDetailsUpdate> detailsPromise);
           Promise<void> abort();
           Promise<boolean> canMakePayment();
 
@@ -824,7 +824,8 @@ 
           p>
         div>
         <p data-tests="payment-request-show-method.https.html">
-          The <a>show()a> method MUST act as follows:
+          The <code><a data-lt="show()">show(detailsPromise)a>code> method
+          MUST act as follows:
         p>
         <ol class="algorithm">
           <li>Let <var>requestvar> be the <a>PaymentRequesta> object on
@@ -937,6 +938,37 @@ 
           <a>DOMExceptiona>, and set the <a>user agenta>'s <a>payment
           request is showinga> boolean to false.
           li>
+          <li>Otherwise, present a user interface that will allow the user to
+          interact with the <var>handlersvar>. The user agent SHOULD
+          prioritize the preference of the user when presenting payment
+          methods.
+          li>
+          <li data-tests=
+          "show-method-optional-promise-rejects-manual.https.html, show-method-optional-promise-resolves-manual.https.html">
+          If <var>detailsPromisevar> was passed, then:
+            <ol>
+              <li>Set <var>requestvar>.<a>[[\updating]]a> to true.
+              li>
+              <li>Disable the user interface user interface that will allow the
+              user to interact with the <var>handlersvar>
+              li>
+              <li>Run the <a>update a <code>PaymentRequestcode>'s details
+              algorithma> with <var>detailsPromisevar> and
+              <var>requestvar>.
+              li>
+              <li>Wait for the <var>detailsPromisevar> to settle.
+                <p class="note">
+                  Based on how the <var>detailsPromisevar> settles, the
+                  <a>update a <code>PaymentRequestcode>'s details
+                  algorithma> determines how the payment UI behaves. That is,
+                  <a>upon rejectiona> of the <var>detailsPromisevar>, the
+                  payment request aborts. Otherwise, <a>upon fulfillmenta>
+                  <var>detailsPromisevar>, the user agent re-enables the
+                  payment request UI and the payment flow can continue.
+                p>
+              li>
+            ol>
+          li>
           <li>
             <p>
               Otherwise, present a user interface to allow the user to interact
@@ -3346,9 +3378,9 @@ 
               <li>Set <var>requestvar>.<a>[[\updating]]a> to false.
               li>
               <li>The <a>user agenta> SHOULD update the user interface based
-              on any changed values in <var>requestvar>. The user agent
-              SHOULD re-enable user interface elements that might have been
-              disabled in the steps above if appropriate.
+              on any changed values in <var>requestvar>. If appropriate, the
+              user agent SHOULD re-enable user interface elements that might
+              have been disabled prior to running this algorithm.
               li>
             ol>
           li>