Generalize textblock merging in joinBackward to handle wrapped single textblocks

FIX: Make `joinBackward` capable of joining textblocks wrapped in parent nodes when the
parent nodes themselves can't be joined (for example two list items which allow only a
single paragraph).

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

Author: marijnh

src/commands.js

@@ -61,9 +61,11 @@ export function joinBackward(state, dispatch, view) {
   return false
 }
 
-function textblockAt(node, side) {
-  for (; node; node = (side == "start" ? node.firstChild : node.lastChild))
+function textblockAt(node, side, only) {
+  for (; node; node = (side == "start" ? node.firstChild : node.lastChild)) {
     if (node.isTextblock) return true
+    if (only && node.childCount != 1) return false
+  }
   return false
 }
 
@@ -405,19 +407,21 @@ function deleteBarrier(state, $cut, dispatch) {
     return true
   }
 
-  if (canDelAfter && after.isTextblock && textblockAt(before, "end")) {
+  if (canDelAfter && textblockAt(after, "start", true) && 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)) {
+    let afterText = after, afterDepth = 1
+    for (; !afterText.isTextblock; afterText = afterText.firstChild) afterDepth++
+    if (at.canReplace(at.childCount, at.childCount, afterText.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,
+                                                     $cut.pos + afterDepth, $cut.pos + after.nodeSize - afterDepth,
                                                      new Slice(end, wrap.length, 0), 0, true))
         dispatch(tr.scrollIntoView())
       }

test/test-commands.js

@@ -75,6 +75,24 @@ describe("joinBackward", () => {
 
   it("does nothing at start of doc", () =>
      apply(doc(p("<a>foo")), joinBackward, null))
+
+  it("can join single-textblock-child nodes", () => {
+    let s = new Schema({
+      nodes: {
+        text: {inline: true},
+        doc: {content: "block+"},
+        block: {content: "para"},
+        para: {content: "text*"}
+      }
+    })
+    let doc = s.node("doc", null, [
+      s.node("block", null, [s.node("para", null, [s.text("a")])]),
+      s.node("block", null, [s.node("para", null, [s.text("b")])])
+    ])
+    let state = EditorState.create({doc, selection: TextSelection.near(doc.resolve(7))})
+    ist(joinBackward(state, tr => state = state.apply(tr)))
+    ist(state.doc.toString(), "doc(block(para(\"ab\")))")
+  })
 })
 
 describe("selectNodeBackward", () => {