Gap cursor for block-edge positions (#335)

A table (or any other block node without a text position of its own) as
the page's first, last, or only block was unreachable from before/after:
neither mouse nor arrow keys could place the cursor there, so no
paragraph could be created around it.

- add the prosemirror-gapcursor plugin as a TipTap extension (via
  @tiptap/pm, no new dependency; schema-neutral, so the editorSchema
  drift fence is unaffected)
- style the gap cursor bar in base.css -- the upstream package does not
  ship its stylesheet through our import path; the blink animation
  honors prefers-reduced-motion
- e2e: keyboard-only round trip that creates paragraphs before and
  after a lone table

Closes #335

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012aoPvnakfBP28nAfijgUY9

apps/web/e2e/editor.spec.ts

@@ -60,6 +60,42 @@ test('typing persists across reload and undo/redo work', async ({ browser }) =>
   await context.close();
 });
 
+test('gap cursor reaches positions before and after a lone table (issue #335)', async ({
+  browser,
+}) => {
+  const context = await contextForUser(browser, BASE_URL, 'fixture-user');
+  const { pondSlug, pageSlug } = await createPage(context, `E2E Gapcursor ${Date.now()}`);
+  const page = await context.newPage();
+
+  await page.goto(`/p/${pondSlug}/${pageSlug}`);
+  await page.getByRole('button', { name: /edit|bearbeiten/i }).click();
+  const status = page.locator('.editor-connection');
+  await expect(status).toHaveAttribute('data-status', 'connected', { timeout: 10000 });
+
+  const content = page.locator('.ProseMirror');
+  await content.click();
+  await page.getByRole('button', { name: /insert table|tabelle einfügen/i }).click();
+  await expect(content.locator('table')).toBeVisible();
+  // Inserting into the empty page replaces the placeholder paragraph — the
+  // table really is the only block, which is the situation of issue #335.
+  await expect(content.locator(':scope > p')).toHaveCount(0);
+
+  // Keyboard only: ArrowUp from the first cell lands on the gap cursor
+  // before the table; typing there materializes a paragraph.
+  await content.locator('th').first().click();
+  await page.keyboard.press('ArrowUp');
+  await page.keyboard.type('above');
+  await expect(content.locator(':scope > :first-child')).toHaveText('above');
+
+  // Same for the position after the table.
+  await content.locator('td').last().click();
+  await page.keyboard.press('ArrowDown');
+  await page.keyboard.type('below');
+  await expect(content.locator(':scope > :last-child')).toHaveText('below');
+
+  await context.close();
+});
+
 test('edit mode hides the sidebar; leaving edit mode restores it', async ({ browser }) => {
   const context = await contextForUser(browser, BASE_URL, 'fixture-user');
   const { pondSlug, pageSlug } = await createPage(context, `E2E Sidebar ${Date.now()}`);

apps/web/src/editor/document-extensions.ts

@@ -1,5 +1,6 @@
 import type { AnyExtension } from '@tiptap/core';
 
+import { GapCursor } from './gap-cursor';
 import { MarkdownClipboard } from './markdown-clipboard';
 import { Bold, CodeMark, Italic, LinkMark, Strikethrough } from './marks';
 import { Image } from './nodes/image';
@@ -63,4 +64,5 @@ export const documentExtensions: AnyExtension[] = [
   Strikethrough,
   LinkMark,
   MarkdownClipboard,
+  GapCursor,
 ];

apps/web/src/editor/gap-cursor.ts

@@ -0,0 +1,20 @@
+import { Extension } from '@tiptap/core';
+import { gapCursor } from '@tiptap/pm/gapcursor';
+
+/**
+ * Cursor position adjacent to block nodes that offer no text position of
+ * their own — without it a table (or code block, image, …) as the page's
+ * first, last, or only block is unreachable from before/after, and no
+ * paragraph can be created there (issue #335). Wraps prosemirror-gapcursor,
+ * which also handles the arrow-key navigation into the gap positions; the
+ * bar itself is styled in `styles/base.css` (`.ProseMirror-gapcursor`)
+ * because the upstream package does not ship its stylesheet through this
+ * entry point.
+ */
+export const GapCursor = Extension.create({
+  name: 'gapCursor',
+
+  addProseMirrorPlugins() {
+    return [gapCursor()];
+  },
+});

apps/web/src/styles/base.css

@@ -1732,6 +1732,42 @@ button {
   outline: none;
 }
 
+/* Gap cursor (issue #335): the blinking bar prosemirror-gapcursor renders at
+   positions adjacent to block nodes without a text position of their own
+   (e.g. a table as the page's only block). The upstream package does not
+   ship its stylesheet through our import path, so these rules replace it. */
+.ProseMirror-gapcursor {
+  display: none;
+  pointer-events: none;
+  position: absolute;
+}
+
+.ProseMirror-gapcursor::after {
+  content: '';
+  display: block;
+  position: absolute;
+  top: -2px;
+  width: 20px;
+  border-top: 1px solid var(--color-text);
+  animation: dt-gapcursor-blink 1.1s steps(2, start) infinite;
+}
+
+@keyframes dt-gapcursor-blink {
+  to {
+    visibility: hidden;
+  }
+}
+
+.ProseMirror-focused .ProseMirror-gapcursor {
+  display: block;
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .ProseMirror-gapcursor::after {
+    animation: none;
+  }
+}
+
 .editor-content h1,
 .editor-content h2,
 .editor-content h3,