diff --git a/web/src/lib/Canvas.svelte b/web/src/lib/Canvas.svelte
index 461478e8..eabfae04 100644
--- a/web/src/lib/Canvas.svelte
+++ b/web/src/lib/Canvas.svelte
@@ -41,13 +41,20 @@
   function scrollsSideways(event: WheelEvent) {
     if (board.viewport.zoom < READABLE) return false;
     for (const target of event.composedPath()) {
-      if (!(target instanceof HTMLElement) || target.classList.contains('svelte-flow__node')) break;
-      const overflow = getComputedStyle(target).overflowX;
-      if ((overflow === 'auto' || overflow === 'scroll') && target.scrollWidth > target.clientWidth + 1) return true;
+      // Nothing outside the window under the pointer scrolls it.
+      if (!(target instanceof HTMLElement)) break;
+      if (target.classList.contains('svelte-flow__node')) break;
+      if (scrollable(target)) return true;
     }
     return false;
   }
 
+  const SCROLLING = new Set(['auto', 'scroll']);
+  function scrollable(element: HTMLElement) {
+    const overflowing = element.scrollWidth > element.clientWidth + 1;
+    return overflowing && SCROLLING.has(getComputedStyle(element).overflowX);
+  }
+
   // A pinch moves a few pixels at a time, a mouse wheel's notch a hundred or
   // so (or a few lines), so a notch is one fixed step rather than a jump.
   const NOTCH = 1.1;