feat(ai-chat): compact reasoning rendering — collapse blank lines (#181)

The "Thinking" (reasoning) block rendered with large vertical gaps: models
emit reasoning with a blank line (\n\n) between every list item and
paragraph, which `marked` turns into loose lists (each <li> wrapped in a
<p>) and separate <p> paragraphs, each carrying a margin.

- Add `collapseBlankLines(text)`: collapse 2+ newlines to one, EXCEPT inside
  fenced code blocks (``` / ~~~) where blank lines are significant. Applied
  in reasoning-block.tsx before renderChatMarkdown, so loose lists become
  tight (no <li><p>) and paragraphs join; `breaks: true` keeps single \n as
  <br>, preserving line breaks. Reasoning-only — the normal answer is
  untouched.
- Drop `white-space: pre-wrap` from `.reasoningText`: on the rendered
  markdown <div> it turned the newlines between block tags into visible
  blank lines on top of the margins. The plain-text fallback <Text> that
  needs pre-wrap already sets it inline.

Tests: collapseBlankLines unit (collapse, fence preservation incl. tilde and
unclosed fences) + rendered-HTML assertions that a blank-line-separated list
becomes a tight list and still parses as a list after a paragraph.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/ai-chat/utils/collapse-blank-lines.test.ts

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+import { describe, it, expect } from "vitest";
+import { collapseBlankLines } from "@/features/ai-chat/utils/collapse-blank-lines.ts";
+import { renderChatMarkdown } from "@/features/ai-chat/utils/markdown.ts";
+
+describe("collapseBlankLines", () => {
+  it("collapses a run of 2+ newlines to a single newline", () => {
+    expect(collapseBlankLines("a\n\nb")).toBe("a\nb");
+    expect(collapseBlankLines("a\n\n\n\nb")).toBe("a\nb");
+  });
+
+  it("keeps single newlines untouched", () => {
+    expect(collapseBlankLines("a\nb\nc")).toBe("a\nb\nc");
+  });
+
+  it("preserves blank lines INSIDE a fenced code block", () => {
+    const src = "a\n\n\nb\n\n```\nx\n\n\ny\n```\n\nc";
+    // Prose blanks collapse; the blank lines between the ``` fences survive.
+    expect(collapseBlankLines(src)).toBe("a\nb\n```\nx\n\n\ny\n```\nc");
+  });
+
+  it("handles a tilde fence and preserves its interior blanks", () => {
+    const src = "p\n\n~~~\ncode\n\nmore\n~~~\n\nq";
+    expect(collapseBlankLines(src)).toBe("p\n~~~\ncode\n\nmore\n~~~\nq");
+  });
+
+  it("leaves an unclosed fence's remaining lines verbatim", () => {
+    const src = "intro\n\n```\nstill\n\nopen";
+    expect(collapseBlankLines(src)).toBe("intro\n```\nstill\n\nopen");
+  });
+
+  it("is a no-op for text with no blank lines", () => {
+    expect(collapseBlankLines("just one line")).toBe("just one line");
+  });
+});
+
+describe("collapseBlankLines + renderChatMarkdown (tight reasoning rendering)", () => {
+  it("renders a blank-line-separated list as a TIGHT list (no <li><p>)", () => {
+    const loose =
+      "Intro paragraph.\n\n- item one\n\n- item two\n\n- item three";
+    const html = renderChatMarkdown(collapseBlankLines(loose), {});
+    // Tight list: each <li> holds the text directly, not wrapped in a <p>.
+    expect(html).toContain("<li>item one</li>");
+    expect(html).not.toContain("<li><p>");
+    // The list still parses as a list after the paragraph (not a paragraph+<br>).
+    expect(html).toContain("<ul>");
+    expect(html).toContain("<p>Intro paragraph.</p>");
+  });
+
+  it("renders an ordered list (1. 2.) as tight after collapsing", () => {
+    const loose = "Intro.\n\n1. first\n\n2. second";
+    const html = renderChatMarkdown(collapseBlankLines(loose), {});
+    expect(html).toContain("<ol>");
+    expect(html).toContain("<li>first</li>");
+    expect(html).not.toContain("<li><p>");
+  });
+
+  it("the loose source WOULD render <li><p> without collapsing (control)", () => {
+    const loose = "- a\n\n- b";
+    expect(renderChatMarkdown(loose, {})).toContain("<li><p>");
+  });
+});

apps/client/src/features/ai-chat/utils/collapse-blank-lines.ts

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+// Pure helper for compact reasoning ("Thinking") rendering. Kept free of React
+// so it can be unit-tested in isolation (see collapse-blank-lines.test.ts).
+
+/**
+ * Collapse runs of 2+ newlines down to a single newline, EXCEPT inside fenced
+ * code blocks (``` ... ``` or ~~~ ... ~~~), where blank lines are significant.
+ *
+ * Why: reasoning models emit thinking with a blank line (`\n\n`) between every
+ * list item and paragraph. `marked` turns those into "loose" lists (each `<li>`
+ * wrapped in a `<p>`) and separate `<p>` paragraphs, each carrying a vertical
+ * margin — so the "Thinking" block renders with large, airy gaps. Removing the
+ * blank-line gaps yields tight lists (no `<li><p>`) and joined paragraphs. The
+ * chat markdown renderer runs with `breaks: true`, so a single `\n` still
+ * becomes a `<br>` — line breaks inside the reasoning are preserved; only the
+ * empty gaps between blocks disappear. Apply ONLY to reasoning text, never to a
+ * normal assistant answer (where paragraph spacing is intentional).
+ *
+ * Fenced code is preserved verbatim: a fence opens on a line whose first
+ * non-space characters are ``` or ~~~ and closes on the next line that starts
+ * with the same fence character. Blank lines between fences (significant for
+ * code formatting) are never collapsed.
+ */
+export function collapseBlankLines(text: string): string {
+  const lines = text.split("\n");
+  const out: string[] = [];
+  let inFence = false;
+  let fenceChar = "";
+
+  for (const line of lines) {
+    const fenceMatch = line.match(/^\s*(`{3,}|~{3,})/);
+    if (fenceMatch) {
+      const ch = fenceMatch[1][0];
+      if (!inFence) {
+        inFence = true;
+        fenceChar = ch;
+      } else if (ch === fenceChar) {
+        inFence = false;
+      }
+      out.push(line);
+      continue;
+    }
+
+    // Inside a fenced block every line (including blanks) is significant.
+    if (inFence) {
+      out.push(line);
+      continue;
+    }
+
+    // Outside fences: drop blank lines so a `\n\n+` gap collapses to a single
+    // `\n` between the surrounding content lines.
+    if (line.trim() === "") continue;
+    out.push(line);
+  }
+
+  return out.join("\n");
+}