diff --git a/web/src/app.css b/web/src/app.css
index 8155e66..cfb3328 100644
--- a/web/src/app.css
+++ b/web/src/app.css
@@ -15,6 +15,15 @@
   gap: 1rem;
   padding: 1.25rem var(--edge) 0;
   font-size: 0.9rem;
+  /* AccountMenu renders nothing at all until auth.status leaves "loading",
+     then either plain text or a .btn-account pill — .btn-account is the
+     tallest thing the bar ever holds, so reserving its height up front is
+     what stops the bar growing the moment a button appears. Derived from
+     .btn-account's own box below rather than guessed: line-height (1.6,
+     inherited unitless from body, resolved against .btn-account's own
+     0.85rem) + its 0.45em top/bottom padding (also relative to 0.85rem) +
+     its 1px border on each edge. Update this if any of those three change. */
+  min-height: calc(1.6 * 0.85rem + 2 * 0.45 * 0.85rem + 2px);
 }
 
 /* A macOS menu-bar split: identity + static nav on the left, the signed-in