diff --git a/src/lib/components/Markdown.svelte b/src/lib/components/Markdown.svelte
index 205fb6a..6786210 100644
--- a/src/lib/components/Markdown.svelte
+++ b/src/lib/components/Markdown.svelte
@@ -108,15 +108,19 @@
     return processed;
   }
 
-  function renderMarkdown(text: string): string {
+  // Two-stage reactive rendering:
+  // Stage 1 — only re-runs when `content` changes (expensive: marked + hljs + spoilers)
+  let parsedHtml = $derived.by(() => {
     try {
-      const html = marked.parse(text) as string;
-      const withSpoilers = processSpoilers(html);
-      return highlightSearchMatches(withSpoilers, searchQuery);
+      const html = marked.parse(content) as string;
+      return processSpoilers(html);
     } catch {
-      return text;
+      return content;
     }
-  }
+  });
+
+  // Stage 2 — re-runs when search changes; skips re-parsing markdown entirely
+  let renderedHtml = $derived(highlightSearchMatches(parsedHtml, searchQuery));
 
   function handleSpoilerClick(event: Event) {
     const target = event.target as HTMLElement;
@@ -191,7 +195,7 @@
   role="presentation"
 >
   <!-- eslint-disable-next-line svelte/no-at-html-tags -- Markdown rendering requires @html; content is from Claude API -->
-  {@html renderMarkdown(content)}
+  {@html renderedHtml}
 </div>
 
 <style>
diff --git a/src/lib/components/Markdown.test.ts b/src/lib/components/Markdown.test.ts
new file mode 100644
index 0000000..56b78c5
--- /dev/null
+++ b/src/lib/components/Markdown.test.ts
@@ -0,0 +1,163 @@
+/**
+ * Markdown Component Tests
+ *
+ * Tests the pure helper functions extracted from the Markdown component:
+ * - processSpoilers: wraps ||text|| syntax in spoiler spans, leaving code blocks untouched
+ * - highlightSearchMatches: injects <mark> tags for search terms, skipping code blocks
+ *
+ * Manual testing checklist:
+ * - [ ] Code blocks render with syntax highlighting and a copy button
+ * - [ ] ||spoiler text|| renders as a hidden span revealed on click
+ * - [ ] Search query highlights matching text in non-code content
+ * - [ ] Links open in the system browser via the Tauri opener
+ */
+
+import { describe, it, expect } from "vitest";
+
+// Mirror functions from Markdown.svelte for isolated testing
+
+function processSpoilers(html: string): string {
+  const codeBlockPlaceholders: string[] = [];
+
+  let processed = html.replace(/<(pre|code)[^>]*>[\s\S]*?<\/\1>/gi, (match) => {
+    codeBlockPlaceholders.push(match);
+    return `__CODE_PLACEHOLDER_${codeBlockPlaceholders.length - 1}__`;
+  });
+
+  processed = processed.replace(
+    /\|\|(.+?)\|\|/g,
+    '<span class="spoiler" role="button" tabindex="0">$1</span>'
+  );
+
+  processed = processed.replace(/__CODE_PLACEHOLDER_(\d+)__/g, (_, index) => {
+    return codeBlockPlaceholders[parseInt(index)];
+  });
+
+  return processed;
+}
+
+function highlightSearchMatches(html: string, query: string): string {
+  if (!query) return html;
+
+  const codeBlockPlaceholders: string[] = [];
+  const tagPlaceholders: string[] = [];
+
+  let processed = html.replace(/<(pre|code)[^>]*>[\s\S]*?<\/\1>/gi, (match) => {
+    codeBlockPlaceholders.push(match);
+    return `__CODE_SEARCH_PLACEHOLDER_${codeBlockPlaceholders.length - 1}__`;
+  });
+
+  processed = processed.replace(/<[^>]+>/g, (match) => {
+    tagPlaceholders.push(match);
+    return `__TAG_PLACEHOLDER_${tagPlaceholders.length - 1}__`;
+  });
+
+  const escapedQuery = query.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+  const regex = new RegExp(`(${escapedQuery})`, "gi");
+  processed = processed.replace(regex, '<mark class="search-highlight">$1</mark>');
+
+  processed = processed.replace(/__TAG_PLACEHOLDER_(\d+)__/g, (_, index) => {
+    return tagPlaceholders[parseInt(index)];
+  });
+
+  processed = processed.replace(/__CODE_SEARCH_PLACEHOLDER_(\d+)__/g, (_, index) => {
+    return codeBlockPlaceholders[parseInt(index)];
+  });
+
+  return processed;
+}
+
+// ---
+
+describe("processSpoilers", () => {
+  it("wraps ||text|| in a spoiler span", () => {
+    const result = processSpoilers("<p>||secret||</p>");
+    expect(result).toContain('<span class="spoiler"');
+    expect(result).toContain("secret");
+  });
+
+  it("adds role=button and tabindex to spoiler spans", () => {
+    const result = processSpoilers("<p>||hidden||</p>");
+    expect(result).toContain('role="button"');
+    expect(result).toContain('tabindex="0"');
+  });
+
+  it("leaves content without spoiler markers unchanged", () => {
+    const html = "<p>Normal text here</p>";
+    expect(processSpoilers(html)).toBe(html);
+  });
+
+  it("handles multiple spoilers in the same string", () => {
+    const result = processSpoilers("<p>||a|| and ||b||</p>");
+    const matches = result.match(/class="spoiler"/g);
+    expect(matches).toHaveLength(2);
+  });
+
+  it("does not apply spoiler syntax inside code blocks", () => {
+    const html = "<pre><code>||not a spoiler||</code></pre>";
+    const result = processSpoilers(html);
+    expect(result).not.toContain('class="spoiler"');
+    expect(result).toContain("||not a spoiler||");
+  });
+
+  it("does not apply spoiler syntax inside inline code", () => {
+    const html = "<p>Example: <code>||inline||</code></p>";
+    const result = processSpoilers(html);
+    expect(result).not.toContain('class="spoiler"');
+  });
+
+  it("handles spoilers adjacent to code blocks correctly", () => {
+    const html = "<pre><code>code</code></pre><p>||revealed||</p>";
+    const result = processSpoilers(html);
+    expect(result).toContain('<span class="spoiler"');
+    expect(result).toContain("<pre><code>code</code></pre>");
+  });
+});
+
+describe("highlightSearchMatches", () => {
+  it("returns unchanged html when query is empty string", () => {
+    const html = "<p>hello world</p>";
+    expect(highlightSearchMatches(html, "")).toBe(html);
+  });
+
+  it("wraps matched text in a mark element", () => {
+    const result = highlightSearchMatches("<p>hello world</p>", "hello");
+    expect(result).toContain('<mark class="search-highlight">hello</mark>');
+  });
+
+  it("is case-insensitive", () => {
+    const result = highlightSearchMatches("<p>Hello World</p>", "hello");
+    expect(result).toContain('<mark class="search-highlight">Hello</mark>');
+  });
+
+  it("highlights all occurrences", () => {
+    const result = highlightSearchMatches("<p>cat and cat</p>", "cat");
+    const matches = result.match(/<mark class="search-highlight">/g);
+    expect(matches).toHaveLength(2);
+  });
+
+  it("does not highlight inside code blocks", () => {
+    const html = "<pre><code>hello inside code</code></pre>";
+    const result = highlightSearchMatches(html, "hello");
+    expect(result).not.toContain('<mark class="search-highlight">');
+    expect(result).toContain("hello inside code");
+  });
+
+  it("does not corrupt HTML tags", () => {
+    const result = highlightSearchMatches('<p class="foo">hello</p>', "hello");
+    expect(result).toContain('<p class="foo">');
+    expect(result).toContain('<mark class="search-highlight">hello</mark>');
+  });
+
+  it("escapes regex special characters in the query", () => {
+    const result = highlightSearchMatches("<p>price: $1.00</p>", "$1");
+    expect(result).toContain('<mark class="search-highlight">$1</mark>');
+  });
+
+  it("highlights text outside code blocks whilst leaving code intact", () => {
+    const html = "<pre><code>match here</code></pre><p>match here too</p>";
+    const result = highlightSearchMatches(html, "match");
+    expect(result).toContain("<pre><code>match here</code></pre>");
+    expect(result).toContain('<mark class="search-highlight">match</mark>');
+  });
+});
diff --git a/src/lib/components/Terminal.svelte b/src/lib/components/Terminal.svelte
index 7e1f59d..a0bc24b 100644
--- a/src/lib/components/Terminal.svelte
+++ b/src/lib/components/Terminal.svelte
@@ -12,15 +12,26 @@
   import { clipboardStore } from "$lib/stores/clipboard";
   import { shouldHidePaths, maskPaths, showThinkingBlocks } from "$lib/stores/config";
 
+  // Virtual windowing constants — keeps the DOM lean during long sessions
+  const WINDOW_SIZE = 150; // max lines rendered at once
+  const LOAD_CHUNK = 50; // how many older lines to load when scrolling up
+  const AVG_LINE_HEIGHT = 60; // rough px estimate per line, used for top spacer
+
   let terminalElement: HTMLDivElement;
   let shouldAutoScroll = true;
   let lines: TerminalLine[] = [];
   let currentSearchQuery = "";
   let currentConversationId: string | null = null;
   let isRestoringScroll = false;
+  let windowStart = 0;
+  let isLoadingMore = false;
 
+  let searchDebounceTimer: ReturnType<typeof setTimeout> | null = null;
   searchQuery.subscribe((value) => {
-    currentSearchQuery = value;
+    if (searchDebounceTimer) clearTimeout(searchDebounceTimer);
+    searchDebounceTimer = setTimeout(() => {
+      currentSearchQuery = value;
+    }, 150);
   });
 
   let hidePaths = false;
@@ -54,12 +65,14 @@
       const savedPosition = claudeStore.getScrollPosition(newId);
       isRestoringScroll = true;
       if (savedPosition === -1) {
-        // Auto-scroll to bottom
+        // Auto-scroll to bottom — window reactive statement will advance windowStart
         shouldAutoScroll = true;
         terminalElement.scrollTop = terminalElement.scrollHeight;
       } else {
-        // Restore to saved position
+        // Restore to saved position — show from the beginning of history
+        windowStart = 0;
         shouldAutoScroll = false;
+        await tick();
         terminalElement.scrollTop = savedPosition;
       }
       // Small delay to prevent the scroll handler from overriding our restore
@@ -69,10 +82,28 @@
     }
   });
 
-  function handleScroll() {
+  async function handleScroll() {
     if (!terminalElement || isRestoringScroll) return;
     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) {
+      isLoadingMore = true;
+      const prevScrollHeight = terminalElement.scrollHeight;
+      const prevScrollTop = terminalElement.scrollTop;
+
+      windowStart = Math.max(0, windowStart - LOAD_CHUNK);
+
+      await tick();
+
+      if (terminalElement) {
+        // Compensate for the new items pushing content down
+        terminalElement.scrollTop =
+          prevScrollTop + (terminalElement.scrollHeight - prevScrollHeight);
+      }
+      isLoadingMore = false;
+    }
   }
 
   afterUpdate(() => {
@@ -138,6 +169,17 @@
     });
   }
 
+  // Visible slice — only render lines within the current window
+  $: visibleLines = lines.slice(windowStart, windowStart + WINDOW_SIZE);
+
+  // 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) {
+    windowStart = Math.max(0, lines.length - WINDOW_SIZE);
+  }
+
   $: {
     if (currentSearchQuery && lines.length > 0) {
       const escapedQuery = currentSearchQuery.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
@@ -179,6 +221,7 @@
     if (terminalElement) {
       terminalElement.removeEventListener("copy", handleCopy);
     }
+    if (searchDebounceTimer) clearTimeout(searchDebounceTimer);
   });
 
   // Copy message content to clipboard
@@ -238,7 +281,13 @@
         Waiting for Claude... Type a message below to start!
       </div>
     {:else}
-      {#each lines as line (line.id)}
+      <div style="height: {topSpacerHeight}px" aria-hidden="true"></div>
+      {#if windowStart > 0}
+        <div class="terminal-older-indicator">
+          ↑ {windowStart} older {windowStart === 1 ? "message" : "messages"} — scroll up to load
+        </div>
+      {/if}
+      {#each visibleLines as line (line.id)}
         {#if line.type === "thinking"}
           {#if showThinking}
             <ThinkingBlock content={line.content} timestamp={line.timestamp} />
@@ -428,6 +477,15 @@
     color: var(--text-secondary);
   }
 
+  .terminal-older-indicator {
+    color: var(--text-tertiary, #6b7280);
+    font-size: 0.75rem;
+    text-align: center;
+    padding: 0.25rem 0;
+    margin-bottom: 0.5rem;
+    border-bottom: 1px solid var(--border-color);
+  }
+
   .terminal-header-text {
     color: var(--text-secondary);
   }
diff --git a/src/lib/components/Terminal.test.ts b/src/lib/components/Terminal.test.ts
index 4d6344d..2343d59 100644
--- a/src/lib/components/Terminal.test.ts
+++ b/src/lib/components/Terminal.test.ts
@@ -89,6 +89,27 @@ function truncateToolContent(content: string): string {
   return content.slice(0, TOOL_COLLAPSE_THRESHOLD) + "…";
 }
 
+// Virtual windowing helpers — mirror the logic in Terminal.svelte
+
+const WINDOW_SIZE = 150;
+const LOAD_CHUNK = 50;
+const AVG_LINE_HEIGHT = 60;
+
+/** Returns the windowStart index when auto-scrolling to the bottom. */
+function autoScrollWindowStart(linesLength: number, windowSize: number): number {
+  return Math.max(0, linesLength - windowSize);
+}
+
+/** Returns the new windowStart after loading LOAD_CHUNK older messages. */
+function olderWindowStart(currentStart: number, chunkSize: number): number {
+  return Math.max(0, currentStart - chunkSize);
+}
+
+/** Returns the height in pixels of the invisible top spacer. */
+function topSpacerHeight(windowStart: number, avgLineHeight: number): number {
+  return windowStart * avgLineHeight;
+}
+
 // ---
 
 describe("getLineClass", () => {
@@ -262,3 +283,86 @@ describe("truncateToolContent", () => {
     expect(result.endsWith("...")).toBe(false);
   });
 });
+
+describe("autoScrollWindowStart", () => {
+  it("returns 0 when lines fit within the window", () => {
+    expect(autoScrollWindowStart(50, WINDOW_SIZE)).toBe(0);
+  });
+
+  it("returns 0 when lines exactly fill the window", () => {
+    expect(autoScrollWindowStart(WINDOW_SIZE, WINDOW_SIZE)).toBe(0);
+  });
+
+  it("advances when lines exceed the window size", () => {
+    expect(autoScrollWindowStart(200, WINDOW_SIZE)).toBe(50);
+  });
+
+  it("never returns a negative value", () => {
+    expect(autoScrollWindowStart(0, WINDOW_SIZE)).toBe(0);
+  });
+
+  it("keeps last WINDOW_SIZE lines visible for large collections", () => {
+    expect(autoScrollWindowStart(500, WINDOW_SIZE)).toBe(350);
+  });
+});
+
+describe("olderWindowStart", () => {
+  it("subtracts the chunk size from the current start", () => {
+    expect(olderWindowStart(100, LOAD_CHUNK)).toBe(50);
+  });
+
+  it("never returns a negative value when chunk is larger than start", () => {
+    expect(olderWindowStart(20, LOAD_CHUNK)).toBe(0);
+  });
+
+  it("returns 0 when current start is 0", () => {
+    expect(olderWindowStart(0, LOAD_CHUNK)).toBe(0);
+  });
+
+  it("returns 0 when current start exactly equals the chunk size", () => {
+    expect(olderWindowStart(LOAD_CHUNK, LOAD_CHUNK)).toBe(0);
+  });
+
+  it("correctly loads a partial chunk near the beginning", () => {
+    expect(olderWindowStart(30, LOAD_CHUNK)).toBe(0);
+  });
+});
+
+describe("topSpacerHeight", () => {
+  it("returns 0 when windowStart is 0", () => {
+    expect(topSpacerHeight(0, AVG_LINE_HEIGHT)).toBe(0);
+  });
+
+  it("multiplies windowStart by avgLineHeight", () => {
+    expect(topSpacerHeight(10, AVG_LINE_HEIGHT)).toBe(600);
+  });
+
+  it("scales linearly with windowStart", () => {
+    expect(topSpacerHeight(50, AVG_LINE_HEIGHT)).toBe(3000);
+    expect(topSpacerHeight(100, AVG_LINE_HEIGHT)).toBe(6000);
+    expect(topSpacerHeight(150, AVG_LINE_HEIGHT)).toBe(9000);
+  });
+
+  it("uses the provided avgLineHeight rather than a hard-coded value", () => {
+    expect(topSpacerHeight(5, 100)).toBe(500);
+    expect(topSpacerHeight(5, 80)).toBe(400);
+  });
+});
+
+describe("virtual windowing constants", () => {
+  it("WINDOW_SIZE is 150", () => {
+    expect(WINDOW_SIZE).toBe(150);
+  });
+
+  it("LOAD_CHUNK is 50", () => {
+    expect(LOAD_CHUNK).toBe(50);
+  });
+
+  it("LOAD_CHUNK is smaller than WINDOW_SIZE", () => {
+    expect(LOAD_CHUNK).toBeLessThan(WINDOW_SIZE);
+  });
+
+  it("AVG_LINE_HEIGHT is a positive number", () => {
+    expect(AVG_LINE_HEIGHT).toBeGreaterThan(0);
+  });
+});
diff --git a/src/lib/stores/achievements.ts b/src/lib/stores/achievements.ts
index 7ae9a37..5e113a0 100644
--- a/src/lib/stores/achievements.ts
+++ b/src/lib/stores/achievements.ts
@@ -1471,11 +1471,14 @@ export const achievementsByRarity = derived(achievementsStore, ($store) => {
   return byRarity;
 });
 
-export const achievementProgress = derived(achievementsStore, ($store) => ({
-  unlocked: $store.totalUnlocked,
-  total: Object.keys($store.achievements).length,
-  percentage: Math.round(($store.totalUnlocked / Object.keys($store.achievements).length) * 100),
-}));
+export const achievementProgress = derived(achievementsStore, ($store) => {
+  const total = Object.keys($store.achievements).length;
+  return {
+    unlocked: $store.totalUnlocked,
+    total,
+    percentage: Math.round(($store.totalUnlocked / total) * 100),
+  };
+});
 
 // Initialize achievement listener
 export async function initAchievementsListener() {