Make any frontmatter <scheme>:// URI's clickable (#1822)

* Make any frontmatter <scheme>:// URI's clickable

http/https links are opened in a new tab/window (via target="_blank"),
other schemes are opened in the current tab, as the browser will prompt
the user to open another application (or do this automatically).

Per https://www.rfc-editor.org/rfc/rfc3986#section-3.1, the scheme must
follow the pattern:

    scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )

* Make `mailto:...` strings clickable in frontmatter

client/codemirror/frontmatter.ts

@@ -12,6 +12,7 @@ import {
   frontmatterQuotesRegex,
   frontmatterUrlRegex,
   frontmatterWikiLinkRegex,
+  frontmatterMailtoRegex,
 } from "../markdown_parser/constants.ts";
 import { processWikiLink, type WikiLinkMatch } from "./wiki_link_processor.ts";
 
@@ -78,7 +79,7 @@ export function frontmatterPlugin(client: Client) {
               const to = from + oMatch[0].length;
               const text = state.sliceDoc(from, to);
 
-              // 1) External links: http(s) URLs
+              // 1) External links: http(s), <scheme>:// URLs
               frontmatterUrlRegex.lastIndex = 0;
               let match: RegExpExecArray | null;
               while ((match = frontmatterUrlRegex.exec(text)) !== null) {
@@ -103,7 +104,14 @@ export function frontmatterPlugin(client: Client) {
                           return;
                         }
                         try {
-                          globalThis.open(url, "_blank");
+                          // Open http(s) links in a new window/tab, open
+                          // alternate schemes in the same page, as they'll
+                          // bounce to another application.
+                          if (/^https?:\/\//i.test(url)) {
+                            globalThis.open(url, "_blank");
+                          } else {
+                            globalThis.open(url, "_self");
+                          }
                         } catch (err) {
                           console.error("Failed to open external link", err);
                         }
@@ -160,6 +168,42 @@ export function frontmatterPlugin(client: Client) {
 
                 widgets.push(...decorations);
               }
+
+              // 3) mailto:... links
+              frontmatterMailtoRegex.lastIndex = 0;
+              let mMatch: RegExpExecArray | null;
+              while ((mMatch = frontmatterMailtoRegex.exec(text)) !== null) {
+                const mFrom = from + (mMatch.index ?? 0);
+                const mTo = mFrom + mMatch[0].length;
+                const url = mMatch[1];
+                const address = url.slice(7);
+                widgets.push(
+                  Decoration.replace({
+                    widget: new LinkWidget({
+                      text: url,
+                      title: `Mail ${address}`,
+                      href: url,
+                      cssClass: "sb-external-link",
+                      from: mFrom,
+                      callback: (e) => {
+                        if (e.altKey) {
+                          // Move cursor into the link
+                          client.editorView.dispatch({
+                            selection: { anchor: mFrom },
+                          });
+                          client.focus();
+                          return;
+                        }
+                        try {
+                          globalThis.open(url, "_self");
+                        } catch (err) {
+                          console.error("Failed to open external link", err);
+                        }
+                      },
+                    }),
+                  }).range(mFrom, mTo),
+                );
+              }
             }
           }
         },

client/markdown_parser/constants.ts

@@ -6,7 +6,8 @@ export const tagRegex =
 export const nakedUrlRegex =
   /(^https?:\/\/([-a-zA-Z0-9@:%_\+~#=]|(?:[.](?!(\s|$)))){1,256})(([-a-zA-Z0-9(@:%_\+~#?&=\/]|(?:[.,:;)](?!(\s|$))))*)/;
 export const frontmatterQuotesRegex = /["'].*["']/g;
-export const frontmatterUrlRegex = /(https?:\/\/[^\s"']+)/g;
+export const frontmatterUrlRegex = /([a-zA-Z][a-zA-Z0-9+.-]*:\/\/[^\s"']+)/g;
 export const frontmatterWikiLinkRegex =
   /(?<leadingTrivia>!?\[\[)(?<stringRef>.*?)(?:\|(?<alias>.*?))?(?<trailingTrivia>\]\])/g;
+export const frontmatterMailtoRegex = /(mailto:[^@\s]+@[^@\s"']+)/ig;
 export const pWikiLinkRegex = new RegExp("^" + wikiLinkRegex.source); // Modified regex used only in parser