Keep asterisks between spaces in comment previews

"2 * 3 * 4" read as emphasis and lost its asterisks. Markdown doesn't
treat a marker with a space just inside it as emphasis.

src/lib/markdownPlain.ts

@@ -9,8 +9,12 @@ export function plainText(body: string): string {
       .replace(/`/gu, "")
       .replace(/~~(.+?)~~/gu, "$1")
       // Emphasis only where a marker sits on a word boundary, so `snake_case`,
-      // `MAX_SIZE` and `_private` survive being read as italics.
-      .replace(/(^|[^\w])[*_]{1,2}([^*_]+?)[*_]{1,2}(?=[^\w]|$)/gu, "$1$2")
+      // `MAX_SIZE` and `_private` survive being read as italics, and with no
+      // space just inside it, so `2 * 3 * 4` does too.
+      .replace(
+        /(^|[^\w])[*_]{1,2}(?=\S)([^*_]+?)(?<=\S)[*_]{1,2}(?=[^\w]|$)/gu,
+        "$1$2",
+      )
       .replace(/^\s{0,3}#{1,6}\s+/gmu, "")
       .replace(/^\s{0,3}>\s?/gmu, "")
       .replace(/^\s{0,3}(?:[-+*]|\d+\.)\s+/gmu, "")

tests/unit/markdownPlain.test.ts

@@ -19,9 +19,13 @@ test.each([
   expect(plainText(markdown)).toBe(expected);
 });
 
-test.each(["snake_case_name", "MAX_SIZE", "_private", "a*b*c"])(
-  "leaves %j alone",
-  text => {
-    expect(plainText(text)).toBe(text);
-  },
-);
+test.each([
+  "snake_case_name",
+  "MAX_SIZE",
+  "_private",
+  "see _foo_bar",
+  "a*b*c",
+  "2 * 3 * 4",
+])("leaves %j alone", text => {
+  expect(plainText(text)).toBe(text);
+});