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](522b1023-6797-401c-9e4a-498570adfc88)

(cherry picked from commit a3cfe6f39ba33cea305de592a006727857014c53)

web_src/js/features/comp/Paste.js

@@ -0,0 +1,154 @@
+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();
+  formData.append('file', file, file.name);
+
+  const res = await POST(uploadUrl, {data: formData});
+  return await res.json();
+}
+
+function triggerEditorContentChanged(target) {
+  target.dispatchEvent(new CustomEvent('ce-editor-content-changed', {bubbles: true}));
+}
+
+class TextareaEditor {
+  constructor(editor) {
+    this.editor = editor;
+  }
+
+  insertPlaceholder(value) {
+    const editor = this.editor;
+    const startPos = editor.selectionStart;
+    const endPos = editor.selectionEnd;
+    editor.value = editor.value.substring(0, startPos) + value + editor.value.substring(endPos);
+    editor.selectionStart = startPos;
+    editor.selectionEnd = startPos + value.length;
+    editor.focus();
+    triggerEditorContentChanged(editor);
+  }
+
+  replacePlaceholder(oldVal, newVal) {
+    const editor = this.editor;
+    const startPos = editor.selectionStart;
+    const endPos = editor.selectionEnd;
+    if (editor.value.substring(startPos, endPos) === oldVal) {
+      editor.value = editor.value.substring(0, startPos) + newVal + editor.value.substring(endPos);
+      editor.selectionEnd = startPos + newVal.length;
+    } else {
+      editor.value = editor.value.replace(oldVal, newVal);
+      editor.selectionEnd -= oldVal.length;
+      editor.selectionEnd += newVal.length;
+    }
+    editor.selectionStart = editor.selectionEnd;
+    editor.focus();
+    triggerEditorContentChanged(editor);
+  }
+}
+
+class CodeMirrorEditor {
+  constructor(editor) {
+    this.editor = editor;
+  }
+
+  insertPlaceholder(value) {
+    const editor = this.editor;
+    const startPoint = editor.getCursor('start');
+    const endPoint = editor.getCursor('end');
+    editor.replaceSelection(value);
+    endPoint.ch = startPoint.ch + value.length;
+    editor.setSelection(startPoint, endPoint);
+    editor.focus();
+    triggerEditorContentChanged(editor.getTextArea());
+  }
+
+  replacePlaceholder(oldVal, newVal) {
+    const editor = this.editor;
+    const endPoint = editor.getCursor('end');
+    if (editor.getSelection() === oldVal) {
+      editor.replaceSelection(newVal);
+    } else {
+      editor.setValue(editor.getValue().replace(oldVal, newVal));
+    }
+    endPoint.ch -= oldVal.length;
+    endPoint.ch += newVal.length;
+    editor.setSelection(endPoint, endPoint);
+    editor.focus();
+    triggerEditorContentChanged(editor.getTextArea());
+  }
+}
+
+async function handleClipboardImages(editor, dropzone, images, e) {
+  const uploadUrl = dropzone.getAttribute('data-upload-url');
+  const filesContainer = dropzone.querySelector('.files');
+
+  if (!dropzone || !uploadUrl || !filesContainer || !images.length) return;
+
+  e.preventDefault();
+  e.stopPropagation();
+
+  for (const img of images) {
+    const name = img.name.slice(0, img.name.lastIndexOf('.'));
+
+    const placeholder = `![${name}](uploading ...)`;
+    editor.insertPlaceholder(placeholder);
+
+    const {uuid} = await uploadFile(img, uploadUrl);
+    const {width, dppx} = await imageInfo(img);
+
+    const url = `/attachments/${uuid}`;
+    let text;
+    if (width > 0 && dppx > 1) {
+      // Scale down images from HiDPI monitors. This uses the <img> tag because it's the only
+      // method to change image size in Markdown that is supported by all implementations.
+      text = `<img width="${Math.round(width / dppx)}" alt="${htmlEscape(name)}" src="${htmlEscape(url)}">`;
+    } else {
+      text = `![${name}](${url})`;
+    }
+    editor.replacePlaceholder(placeholder, text);
+
+    const input = document.createElement('input');
+    input.setAttribute('name', 'files');
+    input.setAttribute('type', 'hidden');
+    input.setAttribute('id', uuid);
+    input.value = uuid;
+    filesContainer.append(input);
+  }
+}
+
+function handleClipboardText(textarea, text, e) {
+  // when pasting links over selected text, turn it into [text](link), except when shift key is held
+  const {value, selectionStart, selectionEnd, _shiftDown} = textarea;
+  if (_shiftDown) return;
+  const selectedText = value.substring(selectionStart, selectionEnd);
+  const trimmedText = text.trim();
+  if (selectedText && isUrl(trimmedText)) {
+    e.stopPropagation();
+    e.preventDefault();
+    replaceTextareaSelection(textarea, `[${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 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);
+    }
+  });
+}