🐛 Fix selectUp off-by-one bug in amp-selector. (ampproject#20889)

Author: caroqliu

extensions/amp-selector/0.1/amp-selector.js

@@ -434,7 +434,12 @@ export class AmpSelector extends AMP.BaseElement {
     // The selection should loop around if the user attempts to go one
     // past the beginning or end.
     const previousIndex = this.elements_.indexOf(this.selectedElements_[0]);
-    const index = previousIndex + delta;
+
+    // If previousIndex === -1 is true, then a negative delta will be offset
+    // one more than is wanted when looping back around in the options.
+    // This occurs when no options are selected and "selectUp" is called.
+    const selectUpWhenNoneSelected = previousIndex === -1 && delta < 0;
+    const index = selectUpWhenNoneSelected ? delta : previousIndex + delta;
     const normalizedIndex = mod(index, this.elements_.length);
 
     this.setSelection_(this.elements_[normalizedIndex]);