utils/react: add preventDefault helpers

These can be used instead of always having to add a comment explaining
what preventDefault does in a given context.

src/utils/react.ts

@@ -0,0 +1,16 @@
+// helper functions for React components
+
+import React from 'react';
+
+/**
+ * Callback that can be passed to onMouseDown event handlers to prevent
+ * an element from becoming focused when clicked.
+ */
+export const preventFocusOnClick: React.MouseEventHandler = (e) => e.preventDefault();
+
+/**
+ * Callback that can be passed to onContextMenu event handlers to prevent
+ * the native browser context menu from being shown.
+ */
+export const preventBrowserNativeContextMenu: React.MouseEventHandler = (e) =>
+    e.preventDefault();