[markdown-preview] fix #6396: resolve relative links properly

Latest version of vscode-uir makes sure that URI always has a scheme  or fall backs to `file` scheme and that URI is always absolute by adding leading `/` to the relative path. It broke an assumption that a URI can be relative in the markdown preview.

Signed-off-by: Anton Kosyakov <anton.kosyakov@typefox.io>

packages/preview/src/browser/markdown/markdown-preview-handler.ts

@@ -18,6 +18,7 @@ import { injectable, inject } from 'inversify';
 import URI from '@theia/core/lib/common/uri';
 import { OpenerService } from '@theia/core/lib/browser';
 import { isOSX } from '@theia/core/lib/common';
+import { Path } from '@theia/core/lib/common/path';
 
 import * as hljs from 'highlight.js';
 import * as markdownit from 'markdown-it';
@@ -91,11 +92,12 @@ export class MarkdownPreviewHandler implements PreviewHandler {
 
     protected resolveUri(link: string, uri: URI, preview: boolean): URI {
         const linkURI = new URI(link);
-        if (!linkURI.path.isAbsolute && (
-            !(linkURI.scheme || linkURI.authority) ||
-            (linkURI.scheme === uri.scheme && linkURI.authority === uri.authority)
-        )) {
-            const resolvedUri = uri.parent.resolve(linkURI.path).withFragment(linkURI.fragment).withQuery(linkURI.query);
+        // URIs are always absolute, check link as a path whether it is relative
+        if (!new Path(link).isAbsolute && linkURI.scheme === uri.scheme &&
+            (!linkURI.authority || linkURI.authority === uri.authority)) {
+            // get a relative path from URI by trimming leading `/`
+            const relativePath = linkURI.path.toString().substring(1);
+            const resolvedUri = uri.parent.resolve(relativePath).withFragment(linkURI.fragment).withQuery(linkURI.query);
             return preview ? PreviewUri.encode(resolvedUri) : resolvedUri;
         }
         return linkURI;

packages/preview/src/browser/preview-uri.ts

@@ -26,7 +26,11 @@ export namespace PreviewUri {
         if (match(uri)) {
             return uri;
         }
-        const query = [param, ...uri.query.split('&')].join('&');
+        const params = [param];
+        if (uri.query) {
+            params.push(...uri.query.split('&'));
+        }
+        const query = params.join('&');
         return uri.withQuery(query);
     }
     export function decode(uri: URI): URI {