Make joinBackward and joinForward able to join lone textblocks into wrapped blocks

... but only if the wrapped textblock is the last child of the wrapping structure.

FIX: `joinBackward` and `joinForward` will now, when the textblock after the cut can't
be moved into the structure before the cut, try to just join the inline content onto the
last child in the structure before the cut.

See https://discuss.prosemirror.net/t/joinbackward-behavior/3296

Author: marijnh

src/commands.js

@@ -379,7 +379,8 @@ function deleteBarrier(state, $cut, dispatch) {
   if (before.type.spec.isolating || after.type.spec.isolating) return false
   if (joinMaybeClear(state, $cut, dispatch)) return true
 
-  if ($cut.parent.canReplace($cut.index(), $cut.index() + 1) &&
+  let canDelAfter = $cut.parent.canReplace($cut.index(), $cut.index() + 1)
+  if (canDelAfter &&
       (conn = (match = before.contentMatchAt(before.childCount)).findWrapping(after.type)) &&
       match.matchType(conn[0] || after.type).validEnd) {
     if (dispatch) {
@@ -402,6 +403,26 @@ function deleteBarrier(state, $cut, dispatch) {
     return true
   }
 
+  if (canDelAfter && after.isTextblock && textblockAt(before, "end")) {
+    let at = before, wrap = []
+    for (;;) {
+      wrap.push(at)
+      if (at.isTextblock) break
+      at = at.lastChild
+    }
+    if (at.canReplace(at.childCount, at.childCount, after.content)) {
+      if (dispatch) {
+        let end = Fragment.empty
+        for (let i = wrap.length - 1; i >= 0; i--) end = Fragment.from(wrap[i].copy(end))
+        let tr = state.tr.step(new ReplaceAroundStep($cut.pos - wrap.length, $cut.pos + after.nodeSize,
+                                                     $cut.pos + 1, $cut.pos + after.nodeSize - 1,
+                                                     new Slice(end, wrap.length, 0), 0, true))
+        dispatch(tr.scrollIntoView())
+      }
+      return true
+    }
+  }
+
   return false
 }