diff --git a/ui/src/components/Header.svelte b/ui/src/components/Header.svelte
index de17518..d050e83 100644
--- a/ui/src/components/Header.svelte
+++ b/ui/src/components/Header.svelte
@@ -57,7 +57,10 @@
     -webkit-user-select: none;
   }
   .nav.grow {
-    flex: 1 1 auto;
+    /* Grow into spare height (banner under the title, the board pinned to the bottom), but
+       never shrink: on a short viewport the banner keeps its height and the board's own
+       scroll (.stage) absorbs the squeeze, instead of the banner and the board splitting it. */
+    flex: 1 0 auto;
   }
   .bar {
     display: flex;