fix(#268): don't open an empty hover card; align flip height estimate (F1,F2)

- F1: gate the card on rows-WITH-text (`thread.some(row => row.text.length > 0)`)
  instead of thread length. A text-less root whose only reply is also text-less
  would otherwise open an empty <Paper> (the render already filters empty rows).
  New test locks it (parent + reply both empty → no card).
- F2: ESTIMATED_CARD_HEIGHT 200 -> 300 (= CARD_MAX_HEIGHT) so the flip-above
  decision reserves the real worst-case height and a tall thread near the
  viewport bottom flips up instead of overflowing off-screen.

vitest 19/19, tsc 0, eslint 0.

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

apps/client/src/features/comment/components/comment-hover-preview.test.tsx

@@ -268,6 +268,33 @@ describe("CommentHoverPreview — hover behaviour", () => {
     expect(card.textContent).toContain("Bob: A reply");
   });
 
+  it("shows nothing when neither the parent nor its reply has any text", () => {
+    // The card is gated on rows-with-text (not thread length), so a text-less
+    // root whose only reply is also text-less must NOT open an empty card.
+    const emptyDoc = JSON.stringify({ type: "doc", content: [] });
+    setComments([
+      comment({
+        id: "c-1",
+        content: emptyDoc,
+        creator: { id: "u-1", name: "Alice", avatarUrl: null } as any,
+      }),
+      comment({
+        id: "c-2",
+        content: emptyDoc,
+        parentCommentId: "c-1",
+        createdAt: new Date(),
+        creator: { id: "u-2", name: "Bob", avatarUrl: null } as any,
+      }),
+    ]);
+    render(<Harness />);
+
+    hoverMark();
+    act(() => {
+      vi.advanceTimersByTime(350);
+    });
+    expect(screen.queryByTestId("comment-hover-preview")).toBeNull();
+  });
+
   it("hides on mouseout", () => {
     setComments([comment()]);
     render(<Harness />);

apps/client/src/features/comment/components/comment-hover-preview.tsx

@@ -18,7 +18,10 @@ const CARD_MAX_WIDTH = 360;
 const CARD_MAX_HEIGHT = 300;
 const GAP = 6;
 // Reserve roughly this much room below the span; flip above when it doesn't fit.
-const ESTIMATED_CARD_HEIGHT = 200;
+// Match CARD_MAX_HEIGHT so the flip-above decision reserves the real worst-case
+// height — otherwise a tall thread placed below near the viewport bottom passes
+// the "fits below" check and then overflows off-screen (clipped, no scroll).
+const ESTIMATED_CARD_HEIGHT = 300;
 
 // One rendered line of the thread: the author and the comment's plain text,
 // pre-computed at hover time so render stays cheap. Shown as "Author: text".
@@ -140,10 +143,10 @@ export default function CommentHoverPreview({
       if (span === activeSpanRef.current) return;
 
       const thread = buildThread(commentMapRef.current, comment);
-      // Show the card when the root has text OR it has at least one reply.
-      // A thread of a single empty-text root carries nothing worth showing.
-      const hasContent =
-        thread.length > 1 || thread.some((row) => row.text.length > 0);
+      // Show the card only when SOME comment has text. Gating on thread length
+      // could open an empty card (a text-less root whose only reply is also
+      // text-less), since the render filters out empty-text rows.
+      const hasContent = thread.some((row) => row.text.length > 0);
       if (!hasContent) return;
 
       activeSpanRef.current = span;