fix(rich-markdown-editor): don't split a list when deleting an empty item

Backspacing an emptied list item in the middle of a list used ProseMirror's default lift, which pulls
the item out into a top-level paragraph — splitting one list into two and stranding an empty paragraph
(a visible gap). Backspace at the start of an empty list item now joins into the previous block instead,
removing the item and keeping a single list. Covers bullet, ordered, and task lists.

Author: waleedlatif1

apps/sim/app/workspace/[workspaceId]/files/components/file-viewer/rich-markdown-editor/keymap.test.ts

@@ -141,3 +141,41 @@ describe('divider Backspace', () => {
     editor.destroy()
   })
 })
+
+describe('empty list-item Backspace', () => {
+  beforeEach(() => {
+    Element.prototype.scrollIntoView = vi.fn()
+  })
+
+  it.each([
+    ['bullet list', '- one\n- two\n- three', 'bulletList'],
+    ['ordered list', '1. one\n2. two\n3. three', 'orderedList'],
+    ['task list', '- [ ] one\n- [ ] two\n- [ ] three', 'taskList'],
+  ])(
+    'joins an empty middle %s item into the previous item instead of splitting the list',
+    (_label, markdown, listType) => {
+      const editor = editorWith('')
+      editor.commands.setContent(markdown, { contentType: 'markdown' })
+      editor.commands.focus()
+      let from = -1
+      let to = -1
+      editor.state.doc.descendants((node, pos) => {
+        if (from < 0 && node.isText && node.text === 'two') {
+          from = pos
+          to = pos + node.text.length
+        }
+      })
+      editor.commands.setTextSelection({ from, to })
+      editor.commands.deleteSelection()
+      pressBackspace(editor)
+
+      expect(blockShape(editor).filter((type) => type === listType)).toEqual([listType])
+      const items: string[] = []
+      editor.state.doc.descendants((node) => {
+        if (node.type.name === 'paragraph' && node.textContent) items.push(node.textContent)
+      })
+      expect(items).toEqual(['one', 'three'])
+      editor.destroy()
+    }
+  )
+})

apps/sim/app/workspace/[workspaceId]/files/components/file-viewer/rich-markdown-editor/keymap.ts

@@ -9,6 +9,9 @@ import { SLASH_COMMAND_PLUGIN_KEY } from './slash-command/slash-command'
 /** Leaf nodes that have no text position, so they can only be reached as a NodeSelection. */
 const SELECTABLE_LEAVES = new Set(['horizontalRule', 'image'])
 
+/** List-item node types whose empty-item Backspace must join rather than lift (see the Backspace docs). */
+const LIST_ITEM_TYPES = new Set(['listItem', 'taskItem'])
+
 /**
  * True while a `/` or `@` suggestion menu is open. Arrow keys must reach that menu's own handler, so
  * the leaf-selection shortcuts below yield rather than stealing the key to select an adjacent divider.
@@ -66,12 +69,15 @@ function selectAdjacentSelectedLeaf(editor: Editor, direction: 'up' | 'down'): b
  * Editor-specific keyboard behavior layered on top of StarterKit's defaults:
  *
  * - **Backspace** at the start of a heading reverts it to a paragraph (ProseMirror's default joins or
- *   no-ops, stranding the heading style; a second Backspace then merges as usual). At the start of a
- *   block whose previous sibling is a divider or image, where ProseMirror's `joinBackward` can't cross
- *   the leaf and no-ops: an *empty* block is deleted (clearing the blank line between/below dividers
- *   without touching the divider itself), while a *non-empty* block selects the leaf — so a first
- *   Backspace highlights what a second deletes, the same highlight-before-delete affordance as clicking
- *   it and parity with the arrow-key leaf selection.
+ *   no-ops, stranding the heading style; a second Backspace then merges as usual). At the start of an
+ *   *empty list item* it joins into the previous block instead of ProseMirror's default lift — lifting
+ *   an empty item out of the middle of a list splits it into two lists and strands an empty paragraph,
+ *   a visible gap; `joinBackward` removes the item and keeps one list. At the start of a block whose
+ *   previous sibling is a divider or image, where ProseMirror's `joinBackward` can't cross the leaf and
+ *   no-ops: an *empty* block is deleted (clearing the blank line between/below dividers without touching
+ *   the divider itself), while a *non-empty* block selects the leaf — so a first Backspace highlights
+ *   what a second deletes, the same highlight-before-delete affordance as clicking it and parity with
+ *   the arrow-key leaf selection.
  * - **Mod-A** inside a code block selects only that block's contents; pressing it again (when the
  *   block is already fully selected) falls through to the default whole-document select-all, the
  *   same scoped behavior as a code editor.
@@ -97,6 +103,12 @@ export const RichMarkdownKeymap = Extension.create({
         if ($from.parent.type.name === 'heading') {
           return editor.commands.setParagraph()
         }
+        if (
+          $from.parent.content.size === 0 &&
+          LIST_ITEM_TYPES.has($from.node($from.depth - 1).type.name)
+        ) {
+          return editor.commands.joinBackward()
+        }
         const blockStart = $from.before($from.depth)
         const nodeBefore = doc.resolve(blockStart).nodeBefore
         if (!nodeBefore || !SELECTABLE_LEAVES.has(nodeBefore.type.name)) return false