fix: correct autoscroll race conditions and lazy load trigger

- Move isRestoringScroll = true before await tick() so scroll events
  fired during the DOM transition cannot incorrectly set shouldAutoScroll
  to false, breaking autoscroll for all tabs except the first
- Guard the windowStart reactive block with isSwitchingConversation so
  stale lines from the previous conversation cannot override the correct
  windowStart after a tab switch
- Fix lazy load threshold to trigger relative to the visible window top
  (windowStart * AVG_LINE_HEIGHT + 300) rather than the absolute scroll
  container top, so older messages load without scrolling through the spacer

src/lib/components/Terminal.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
   import { claudeStore, type TerminalLine } from "$lib/stores/claude";
   import { afterUpdate, tick, onMount, onDestroy } from "svelte";
+  import { get } from "svelte/store";
   import { invoke } from "@tauri-apps/api/core";
 
   import ConversationTabs from "./ConversationTabs.svelte";
@@ -25,6 +26,7 @@
   let isRestoringScroll = false;
   let windowStart = 0;
   let isLoadingMore = false;
+  let isSwitchingConversation = false;
 
   let searchDebounceTimer: ReturnType<typeof setTimeout> | null = null;
   searchQuery.subscribe((value) => {
@@ -59,24 +61,39 @@
 
     currentConversationId = newId;
 
-    // Peek at the saved position to set windowStart before the first tick,
-    // preventing a stale windowStart from a previous conversation leaving
-    // visibleLines empty (windowStart >= lines.length).
+    // Guard the $: reactive auto-scroll block from firing with stale `lines`
+    // (the old conversation's data) during the switch. Without this, Svelte's
+    // reactive system can re-run the window-advance block before `terminalLines`
+    // has recomputed for the new conversation, overriding our correct windowStart.
+    isSwitchingConversation = true;
+
+    // Read the new conversation's lines directly from the store — the derived
+    // `terminalLines` store (and thus `lines`) may not have recomputed yet when
+    // this subscriber fires, so using `lines` here would give stale data.
+    const newConvLines = get(claudeStore.conversations).get(newId)?.terminalLines ?? [];
     const savedPosition = claudeStore.getScrollPosition(newId);
     if (savedPosition === -1) {
       // Will auto-scroll: pin the window to the tail of the new conversation
       shouldAutoScroll = true;
-      windowStart = Math.max(0, lines.length - WINDOW_SIZE);
+      windowStart = Math.max(0, newConvLines.length - WINDOW_SIZE);
     } else {
       // Will restore a specific position: always start from the top of history
       shouldAutoScroll = false;
       windowStart = 0;
     }
 
-    // Restore scroll position for the new conversation after DOM updates
+    // Block the scroll handler during the entire DOM transition — scroll events
+    // can fire mid-tick when the content changes, and handleScroll would see
+    // scrollTop not at the bottom yet and set shouldAutoScroll = false, breaking
+    // autoscroll for the new conversation permanently.
+    isRestoringScroll = true;
+
+    // Restore scroll position for the new conversation after DOM updates.
+    // Clear the switch guard first so the $: block can react to new lines
+    // arriving after the switch settles.
     await tick();
+    isSwitchingConversation = false;
     if (terminalElement) {
-      isRestoringScroll = true;
       if (savedPosition === -1) {
         terminalElement.scrollTop = terminalElement.scrollHeight;
       } else {
@@ -94,8 +111,10 @@
     const { scrollTop, scrollHeight, clientHeight } = terminalElement;
     shouldAutoScroll = scrollHeight - scrollTop - clientHeight < 100;
 
-    // Load older lines when the user scrolls near the top
-    if (scrollTop < 300 && windowStart > 0 && !isLoadingMore) {
+    // Load older lines when the user scrolls near the top of the visible window.
+    // Use windowStart * AVG_LINE_HEIGHT (the spacer height) as the baseline so
+    // we trigger at the top of the rendered content, not the absolute container top.
+    if (scrollTop < windowStart * AVG_LINE_HEIGHT + 300 && windowStart > 0 && !isLoadingMore) {
       isLoadingMore = true;
       const prevScrollHeight = terminalElement.scrollHeight;
       const prevScrollTop = terminalElement.scrollTop;
@@ -182,8 +201,10 @@
   // Height of the invisible spacer above the visible window
   $: topSpacerHeight = windowStart * AVG_LINE_HEIGHT;
 
-  // Advance the window forward when auto-scrolling and new lines overflow it
-  $: if (shouldAutoScroll && lines.length > windowStart + WINDOW_SIZE) {
+  // Advance the window forward when auto-scrolling and new lines overflow it.
+  // Skip during conversation switches — `lines` may still hold the previous
+  // conversation's data, which would push windowStart past the new conv's end.
+  $: if (shouldAutoScroll && !isSwitchingConversation && lines.length > windowStart + WINDOW_SIZE) {
     windowStart = Math.max(0, lines.length - WINDOW_SIZE);
   }