fix: size mermaid iframe by the diagram's aspect ratio (#40046)

Mermaid scales diagrams wider than the markup column down to its width,
but the iframe kept the unscaled viewBox height, leaving a large blank
area below them.

The iframe now uses a CSS `aspect-ratio` capped at the viewBox height,
so it follows the rendered diagram on any width and resize without
measuring, which keeps diagrams in collapsed `<details>` working.
Diagrams that don't scale (`useMaxWidth: false`) keep the fixed height.

Fixes: https://github.com/go-gitea/gitea/issues/40045

tests/e2e/mermaid.test.ts

@@ -6,16 +6,23 @@ test('mermaid diagram in issue', async ({page, request}) => {
   const repoName = `e2e-mermaid-${randomString(8)}`;
   const owner = env.GITEA_TEST_E2E_USER;
   await apiCreateRepo(request, {name: repoName, autoInit: false});
-  const body = '```mermaid\nflowchart LR\n  Alpha --> Beta\n  Beta --> Gamma\n```\n';
-  const {index} = await apiCreateIssue(request, {owner, repo: repoName, title: 'mermaid test', body});
+  const source = 'flowchart TD\n  Alpha --> Beta --> Gamma\n  Alpha --> Node1 & Node2 & Node3 & Node4 & Node5 & Node6 & Node7 & Node8';
+  const {index} = await apiCreateIssue(request, {owner, repo: repoName, title: 'mermaid test', body: `\`\`\`mermaid\n${source}\n\`\`\`\n`});
   await page.goto(`/${owner}/${repoName}/issues/${index}`);
 
   const svg = page.frameLocator('iframe.markup-content-iframe').locator('svg');
   await expect(svg).toContainText(/Alpha[\s\S]*Beta[\s\S]*Gamma/);
+  const [iframeWidth, iframeHeight, svgWidth, svgHeight] = await page.locator('iframe.markup-content-iframe').evaluate((iframe: HTMLIFrameElement) => {
+    const iframeRect = iframe.getBoundingClientRect();
+    const svgRect = iframe.contentDocument!.querySelector('svg')!.getBoundingClientRect();
+    return [iframeRect.width, iframeRect.height, svgRect.width, svgRect.height];
+  });
+  expect(svgWidth).toBeCloseTo(iframeWidth, 0);
+  expect(svgHeight).toBeCloseTo(iframeHeight, 0);
 
   await svg.hover();
   await page.locator('.code-copy').click();
-  await expect.poll(() => page.evaluate(() => navigator.clipboard.readText())).toBe('flowchart LR\n  Alpha --> Beta\n  Beta --> Gamma');
+  await expect.poll(() => page.evaluate(() => navigator.clipboard.readText())).toBe(source);
 
   await assertNoJsError(page);
 });

web_src/js/markup/mermaid.ts

@@ -111,8 +111,16 @@ export async function initMarkupCodeMermaid(elMarkup: HTMLElement): Promise<void
       const iframe = document.createElement('iframe');
       iframe.classList.add('markup-content-iframe', 'tw-invisible');
       iframe.srcdoc = html`<style>body { margin: 0; overflow: hidden; } #mermaid { display: block; margin: 0 auto; }</style>`;
-      const viewBoxHeight = svgNode.viewBox.baseVal?.height;
-      if (viewBoxHeight) iframe.style.height = `${Math.max(Math.ceil(viewBoxHeight), 85)}px`; // min-height keeps the buttons from overlapping
+      const viewBox = svgNode.viewBox.baseVal;
+      if (viewBox?.height) {
+        iframe.style.height = `${Math.ceil(viewBox.height)}px`;
+        iframe.style.minHeight = '85px'; // keeps the buttons from overlapping
+        if (svgNode.getAttribute('width') === '100%') { // svg shrinks to the iframe width, so the height must follow its aspect ratio
+          iframe.style.aspectRatio = `${viewBox.width} / ${viewBox.height}`;
+          iframe.style.maxHeight = iframe.style.height;
+          iframe.style.height = 'auto';
+        }
+      }
 
       // the iframe will be fully reloaded if its DOM context is changed (e.g.: moved in the DOM tree).
       // to avoid unnecessary reloading, we should insert the iframe to its final position only once.