diff --git a/src/lib/components/ConversationTabs.svelte b/src/lib/components/ConversationTabs.svelte
index 75fe9e1..d45a4b8 100644
--- a/src/lib/components/ConversationTabs.svelte
+++ b/src/lib/components/ConversationTabs.svelte
@@ -216,6 +216,14 @@
 
   // Keyboard shortcuts
   onMount(() => {
+    // Initialise all conversations as seen on mount so that remounting
+    // this component (e.g. after closing the file editor) doesn't falsely
+    // mark existing messages as unread.
+    for (const [id, conversation] of $conversations) {
+      lastSeenMessageCount.set(id, conversation.terminalLines.length);
+    }
+    lastSeenMessageCount = lastSeenMessageCount;
+
     function handleGlobalKeydown(event: KeyboardEvent) {
       // Ctrl/Cmd + T: New tab
       if ((event.ctrlKey || event.metaKey) && event.key === "t") {