fix: Improve backspace handling (#2284), fix #2281

* improve backspace handling

* revert codeblock changes

* revert codeblock changes

* fix tests

Co-authored-by: Philipp Kühn <philippkuehn@MacBook-Pro-von-Philipp.local>

packages/core/src/extensions/keymap.ts

@@ -1,3 +1,6 @@
+import { Plugin, PluginKey, Selection } from 'prosemirror-state'
+import { createChainableState } from '../helpers/createChainableState'
+import { CommandManager } from '../CommandManager'
 import { Extension } from '../Extension'
 
 export const Keymap = Extension.create({
@@ -6,6 +9,24 @@ export const Keymap = Extension.create({
   addKeyboardShortcuts() {
     const handleBackspace = () => this.editor.commands.first(({ commands }) => [
       () => commands.undoInputRule(),
+      // maybe convert first text block node to default node
+      () => commands.command(({ tr }) => {
+        const { selection, doc } = tr
+        const { empty, $anchor } = selection
+        const { pos, parent } = $anchor
+        const isAtStart = Selection.atStart(doc).from === pos
+
+        if (
+          !empty
+          || !isAtStart
+          || !parent.type.isTextblock
+          || parent.textContent.length
+        ) {
+          return false
+        }
+
+        return commands.clearNodes()
+      }),
       () => commands.deleteSelection(),
       () => commands.joinBackward(),
       () => commands.selectNodeBackward(),
@@ -33,4 +54,53 @@ export const Keymap = Extension.create({
       'Mod-a': () => this.editor.commands.selectAll(),
     }
   },
+
+  addProseMirrorPlugins() {
+    return [
+      // With this plugin we check if the whole document was selected and deleted.
+      // In this case we will additionally call `clearNodes()` to convert e.g. a heading
+      // to a paragraph if necessary.
+      // This is an alternative to ProseMirror's `AllSelection`, which doesn’t work well
+      // with many other commands.
+      new Plugin({
+        key: new PluginKey('clearDocument'),
+        appendTransaction: (transactions, oldState, newState) => {
+          const docChanges = transactions.some(transaction => transaction.docChanged)
+            && !oldState.doc.eq(newState.doc)
+
+          if (!docChanges) {
+            return
+          }
+
+          const { empty, from, to } = oldState.selection
+          const allFrom = Selection.atStart(oldState.doc).from
+          const allEnd = Selection.atEnd(oldState.doc).to
+          const allWasSelected = from === allFrom && to === allEnd
+          const isEmpty = newState.doc.textBetween(0, newState.doc.content.size, ' ', ' ').length === 0
+
+          if (empty || !allWasSelected || !isEmpty) {
+            return
+          }
+
+          const tr = newState.tr
+          const state = createChainableState({
+            state: newState,
+            transaction: tr,
+          })
+          const { commands } = new CommandManager({
+            editor: this.editor,
+            state,
+          })
+
+          commands.clearNodes()
+
+          if (!tr.steps.length) {
+            return
+          }
+
+          return tr
+        },
+      }),
+    ]
+  },
 })