Improve desktop command autocomplete to match combobox behavior (#53)

* feat(app): align command autocomplete with desktop combobox

* refactor(app): extract shared autocomplete UI and command hook

* refactor(app): rename autocomplete hooks to generic naming

packages/app/src/components/ui/autocomplete-utils.ts

@@ -0,0 +1,56 @@
+import { getNextActiveIndex } from "./combobox-keyboard";
+
+export type AutocompleteOptionsPosition = "above-input" | "below-input";
+
+export function orderAutocompleteOptions<T>(
+  options: readonly T[],
+  position: AutocompleteOptionsPosition = "above-input"
+): T[] {
+  if (position === "below-input") {
+    return [...options];
+  }
+  return [...options].reverse();
+}
+
+export function getAutocompleteFallbackIndex(
+  itemCount: number,
+  position: AutocompleteOptionsPosition = "above-input"
+): number {
+  if (itemCount <= 0) {
+    return -1;
+  }
+  return position === "above-input" ? itemCount - 1 : 0;
+}
+
+export function getAutocompleteNextIndex(args: {
+  currentIndex: number;
+  itemCount: number;
+  key: "ArrowDown" | "ArrowUp";
+}): number {
+  return getNextActiveIndex(args);
+}
+
+export function getAutocompleteScrollOffset(args: {
+  currentOffset: number;
+  viewportHeight: number;
+  itemTop: number;
+  itemHeight: number;
+}): number {
+  if (args.viewportHeight <= 0) {
+    return args.currentOffset;
+  }
+
+  const itemBottom = args.itemTop + args.itemHeight;
+  const viewportTop = args.currentOffset;
+  const viewportBottom = args.currentOffset + args.viewportHeight;
+
+  if (args.itemTop < viewportTop) {
+    return Math.max(0, args.itemTop);
+  }
+
+  if (itemBottom > viewportBottom) {
+    return Math.max(0, itemBottom - args.viewportHeight);
+  }
+
+  return args.currentOffset;
+}