Improvements for Content Copy (#21842)

It now supports copying Markdown, SVG and Images (not in Firefox
currently because of lacking
[`ClipboardItem`](https://developer.mozilla.org/en-US/docs/Web/API/ClipboardItem)
support, but can be enabled in `about:config` and works). It will fetch
the data if in a rendered view or when it's an image.

Followup to https://github.com/go-gitea/gitea/pull/21629.

web_src/js/features/copycontent.js

@@ -0,0 +1,59 @@
+import {copyToClipboard} from './clipboard.js';
+import {showTemporaryTooltip} from '../modules/tippy.js';
+import {convertImage} from '../utils.js';
+const {i18n} = window.config;
+
+async function doCopy(content, btn) {
+  const success = await copyToClipboard(content);
+  showTemporaryTooltip(btn, success ? i18n.copy_success : i18n.copy_error);
+}
+
+export function initCopyContent() {
+  const btn = document.getElementById('copy-content');
+  if (!btn || btn.classList.contains('disabled')) return;
+
+  btn.addEventListener('click', async () => {
+    if (btn.classList.contains('is-loading')) return;
+    let content, isImage;
+    const link = btn.getAttribute('data-link');
+
+    // when data-link is present, we perform a fetch. this is either because
+    // the text to copy is not in the DOM or it is an image which should be
+    // fetched to copy in full resolution
+    if (link) {
+      btn.classList.add('is-loading');
+      try {
+        const res = await fetch(link, {credentials: 'include', redirect: 'follow'});
+        const contentType = res.headers.get('content-type');
+
+        if (contentType.startsWith('image/') && !contentType.startsWith('image/svg')) {
+          isImage = true;
+          content = await res.blob();
+        } else {
+          content = await res.text();
+        }
+      } catch {
+        return showTemporaryTooltip(btn, i18n.copy_error);
+      } finally {
+        btn.classList.remove('is-loading');
+      }
+    } else { // text, read from DOM
+      const lineEls = document.querySelectorAll('.file-view .lines-code');
+      content = Array.from(lineEls).map((el) => el.textContent).join('');
+    }
+
+    try {
+      await doCopy(content, btn);
+    } catch {
+      if (isImage) { // convert image to png as last-resort as some browser only support png copy
+        try {
+          await doCopy(await convertImage(content, 'image/png'), btn);
+        } catch {
+          showTemporaryTooltip(btn, i18n.copy_error);
+        }
+      } else {
+        showTemporaryTooltip(btn, i18n.copy_error);
+      }
+    }
+  });
+}