Support pasting URLs over markdown text (#29566)

Support pasting URLs over selection text in the textarea editor. Does
not work in EasyMDE and I don't intend to support it. Image paste works
as usual in both Textarea and EasyMDE.

The new `replaceTextareaSelection` function changes textarea content via
[`insertText`](https://developer.mozilla.org/en-US/docs/Web/API/Document/execCommand#using_inserttext)
command, which preserves history, e.g. `CTRL-Z` works and is also
demostrated below. We should later refactor the image paste code to use
the same function because it currently destroys history.

Overriding the formatting via `Shift` key is supported as well, e.g.
`Ctrl+Shift+V` will insert the URL as-is, like on GitHub.


![urlpaste](https://github.com/go-gitea/gitea/assets/115237/522b1023-6797-401c-9e4a-498570adfc88)

5 files changed, 103 insertions, 31 deletions

diff --git a/web_src/js/features/comp/ComboMarkdownEditor.js b/web_src/js/features/comp/ComboMarkdownEditor.js
index 4c973358e3..1e7b554b98 100644
--- a/web_src/js/features/comp/ComboMarkdownEditor.js
+++ b/web_src/js/features/comp/ComboMarkdownEditor.js
@@ -3,7 +3,7 @@ import '@github/text-expander-element';
 import $ from 'jquery';
 import {attachTribute} from '../tribute.js';
 import {hideElem, showElem, autosize, isElemVisible} from '../../utils/dom.js';
-import {initEasyMDEImagePaste, initTextareaImagePaste} from './ImagePaste.js';
+import {initEasyMDEPaste, initTextareaPaste} from './Paste.js';
 import {handleGlobalEnterQuickSubmit} from './QuickSubmit.js';
 import {renderPreviewPanelContent} from '../repo-editor.js';
 import {easyMDEToolbarActions} from './EasyMDEToolbarActions.js';
@@ -84,6 +84,17 @@ class ComboMarkdownEditor {
       if (el.nodeName === 'BUTTON' && !el.getAttribute('type')) el.setAttribute('type', 'button');
     }
 
+    this.textarea.addEventListener('keydown', (e) => {
+      if (e.shiftKey) {
+        e.target._shiftDown = true;
+      }
+    });
+    this.textarea.addEventListener('keyup', (e) => {
+      if (!e.shiftKey) {
+        e.target._shiftDown = false;
+      }
+    });
+
     const monospaceButton = this.container.querySelector('.markdown-switch-monospace');
     const monospaceEnabled = localStorage?.getItem('markdown-editor-monospace') === 'true';
     const monospaceText = monospaceButton.getAttribute(monospaceEnabled ? 'data-disable-text' : 'data-enable-text');
@@ -108,7 +119,7 @@ class ComboMarkdownEditor {
     });
 
     if (this.dropzone) {
-      initTextareaImagePaste(this.textarea, this.dropzone);
+      initTextareaPaste(this.textarea, this.dropzone);
     }
   }
 
@@ -241,7 +252,7 @@ class ComboMarkdownEditor {
     });
     this.applyEditorHeights(this.container.querySelector('.CodeMirror-scroll'), this.options.editorHeights);
     await attachTribute(this.easyMDE.codemirror.getInputField(), {mentions: true, emoji: true});
-    initEasyMDEImagePaste(this.easyMDE, this.dropzone);
+    initEasyMDEPaste(this.easyMDE, this.dropzone);
     hideElem(this.textareaMarkdownToolbar);
   }
 
diff --git a/web_src/js/features/comp/ImagePaste.js b/web_src/js/features/comp/Paste.js
index b727880bc8..b26296d1fc 100644
--- a/web_src/js/features/comp/ImagePaste.js
+++ b/web_src/js/features/comp/Paste.js
@@ -1,6 +1,8 @@
 import {htmlEscape} from 'escape-goat';
 import {POST} from '../../modules/fetch.js';
 import {imageInfo} from '../../utils/image.js';
+import {getPastedContent, replaceTextareaSelection} from '../../utils/dom.js';
+import {isUrl} from '../../utils/url.js';
 
 async function uploadFile(file, uploadUrl) {
   const formData = new FormData();
@@ -10,17 +12,6 @@ async function uploadFile(file, uploadUrl) {
   return await res.json();
 }
 
-function clipboardPastedImages(e) {
-  if (!e.clipboardData) return [];
-
-  const files = [];
-  for (const item of e.clipboardData.items || []) {
-    if (!item.type || !item.type.startsWith('image/')) continue;
-    files.push(item.getAsFile());
-  }
-  return files;
-}
-
 function triggerEditorContentChanged(target) {
   target.dispatchEvent(new CustomEvent('ce-editor-content-changed', {bubbles: true}));
 }
@@ -91,20 +82,16 @@ class CodeMirrorEditor {
   }
 }
 
-const uploadClipboardImage = async (editor, dropzone, e) => {
+async function handleClipboardImages(editor, dropzone, images, e) {
   const uploadUrl = dropzone.getAttribute('data-upload-url');
   const filesContainer = dropzone.querySelector('.files');
 
-  if (!uploadUrl || !filesContainer) return;
+  if (!dropzone || !uploadUrl || !filesContainer || !images.length) return;
 
-  const pastedImages = clipboardPastedImages(e);
-  if (!pastedImages || pastedImages.length === 0) {
-    return;
-  }
   e.preventDefault();
   e.stopPropagation();
 
-  for (const img of pastedImages) {
+  for (const img of images) {
     const name = img.name.slice(0, img.name.lastIndexOf('.'));
 
     const placeholder = `![${name}](uploading ...)`;
@@ -131,18 +118,37 @@ const uploadClipboardImage = async (editor, dropzone, e) => {
     input.value = uuid;
     filesContainer.append(input);
   }
-};
+}
 
-export function initEasyMDEImagePaste(easyMDE, dropzone) {
-  if (!dropzone) return;
-  easyMDE.codemirror.on('paste', async (_, e) => {
-    return uploadClipboardImage(new CodeMirrorEditor(easyMDE.codemirror), dropzone, e);
+function handleClipboardText(textarea, text, e) {
+  // when pasting links over selected text, turn it into [text](link), except when shift key is held
+  const {value, selectionStart, selectionEnd, _shiftDown} = textarea;
+  if (_shiftDown) return;
+  const selectedText = value.substring(selectionStart, selectionEnd);
+  const trimmedText = text.trim();
+  if (selectedText && isUrl(trimmedText)) {
+    e.stopPropagation();
+    e.preventDefault();
+    replaceTextareaSelection(textarea, `[${selectedText}](${trimmedText})`);
+  }
+}
+
+export function initEasyMDEPaste(easyMDE, dropzone) {
+  easyMDE.codemirror.on('paste', (_, e) => {
+    const {images} = getPastedContent(e);
+    if (images.length) {
+      handleClipboardImages(new CodeMirrorEditor(easyMDE.codemirror), dropzone, images, e);
+    }
   });
 }
 
-export function initTextareaImagePaste(textarea, dropzone) {
-  if (!dropzone) return;
-  textarea.addEventListener('paste', async (e) => {
-    return uploadClipboardImage(new TextareaEditor(textarea), dropzone, e);
+export function initTextareaPaste(textarea, dropzone) {
+  textarea.addEventListener('paste', (e) => {
+    const {images, text} = getPastedContent(e);
+    if (images.length) {
+      handleClipboardImages(new TextareaEditor(textarea), dropzone, images, e);
+    } else if (text) {
+      handleClipboardText(textarea, text, e);
+    }
   });
 }
diff --git a/web_src/js/utils/dom.js b/web_src/js/utils/dom.js
index 91535dc187..aa7c2604aa 100644
--- a/web_src/js/utils/dom.js
+++ b/web_src/js/utils/dom.js
@@ -243,3 +243,39 @@ export function isElemVisible(element) {
 
   return Boolean(element.offsetWidth || element.offsetHeight || element.getClientRects().length);
 }
+
+// extract text and images from "paste" event
+export function getPastedContent(e) {
+  const images = [];
+  for (const item of e.clipboardData?.items ?? []) {
+    if (item.type?.startsWith('image/')) {
+      images.push(item.getAsFile());
+    }
+  }
+  const text = e.clipboardData?.getData?.('text') ?? '';
+  return {text, images};
+}
+
+// replace selected text in a textarea while preserving editor history, e.g. CTRL-Z works after this
+export function replaceTextareaSelection(textarea, text) {
+  const before = textarea.value.slice(0, textarea.selectionStart ?? undefined);
+  const after = textarea.value.slice(textarea.selectionEnd ?? undefined);
+  let success = true;
+
+  textarea.contentEditable = 'true';
+  try {
+    success = document.execCommand('insertText', false, text);
+  } catch {
+    success = false;
+  }
+  textarea.contentEditable = 'false';
+
+  if (success && !textarea.value.slice(0, textarea.selectionStart ?? undefined).endsWith(text)) {
+    success = false;
+  }
+
+  if (!success) {
+    textarea.value = `${before}${text}${after}`;
+    textarea.dispatchEvent(new CustomEvent('change', {bubbles: true, cancelable: true}));
+  }
+}
diff --git a/web_src/js/utils/url.js b/web_src/js/utils/url.js
index a40737ca6f..470ece31b0 100644
--- a/web_src/js/utils/url.js
+++ b/web_src/js/utils/url.js
@@ -1,3 +1,15 @@
 export function pathEscapeSegments(s) {
   return s.split('/').map(encodeURIComponent).join('/');
 }
+
+function stripSlash(url) {
+  return url.endsWith('/') ? url.slice(0, -1) : url;
+}
+
+export function isUrl(url) {
+  try {
+    return stripSlash((new URL(url).href)).trim() === stripSlash(url).trim();
+  } catch {
+    return false;
+  }
+}
diff --git a/web_src/js/utils/url.test.js b/web_src/js/utils/url.test.js
index 3dbedec94f..08c6373ffb 100644
--- a/web_src/js/utils/url.test.js
+++ b/web_src/js/utils/url.test.js
@@ -1,6 +1,13 @@
-import {pathEscapeSegments} from './url.js';
+import {pathEscapeSegments, isUrl} from './url.js';
 
 test('pathEscapeSegments', () => {
   expect(pathEscapeSegments('a/b/c')).toEqual('a/b/c');
   expect(pathEscapeSegments('a/b/ c')).toEqual('a/b/%20c');
 });
+
+test('isUrl', () => {
+  expect(isUrl('https://example.com')).toEqual(true);
+  expect(isUrl('https://example.com/')).toEqual(true);
+  expect(isUrl('https://example.com/index.html')).toEqual(true);
+  expect(isUrl('/index.html')).toEqual(false);
+});