Ensure confirmation dialog appears above top bar

Set a Z-index that is sufficient to make the dialog appear on top of
other content in the sidebar, mainly the top bar. We will likely have to
revise this if `confirm` is used in other contexts in future.

src/shared/prompts.js

@@ -35,6 +35,12 @@ export async function confirm({
 
   const container = document.createElement('div');
   container.setAttribute('data-testid', 'confirm-container');
+
+  // Ensure dialog appears above any existing content. The Z-index value here
+  // is Good Enough™ for current usage.
+  container.style.position = 'relative';
+  container.style.zIndex = '10';
+
   document.body.appendChild(container);
 
   return new Promise(resolve => {