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src/automats/userSettingsAutomat.js

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+import { Automat } from 'react-automat';
+
+/**
+ * @typedef {'top'|'right'|'below'} ViewerPosition
+ */
+
+/**
+ * @typedef {Object} UserSettingsState
+ * @property {ViewerPosition} viewerPosition Preferred docking position of the
+ *   attachment viewer (text editor / PDF preview) relative to the article list.
+ */
+
+/** @type {UserSettingsState} */
+const initialState = {
+  viewerPosition: 'top',
+};
+
+/**
+ * Persisted per-user UI preferences. Unlike the other automats this one is
+ * backed by IndexedDB (`persist: true`) so settings survive reloads and
+ * sessions.
+ *
+ * @type {Automat<UserSettingsState>}
+ */
+const userSettingsAutomat = new Automat(
+  initialState,
+  {
+    /**
+     * Set the docking position of the attachment viewer.
+     *
+     * @param {ViewerPosition} position One of `'top'`, `'right'` or `'below'`.
+     * @returns {void}
+     */
+    setViewerPosition(position) {
+      if (position !== 'top' && position !== 'right' && position !== 'below') {
+        return;
+      }
+      userSettingsAutomat.setState({ viewerPosition: position });
+    },
+  },
+  {
+    name: 'userSettings',
+    persist: true,
+  }
+);
+
+export default userSettingsAutomat;
+export const { setViewerPosition } = userSettingsAutomat.actions;