Prevent deleteMarkupBackward from acting on continued paragraphs

FIX: Fix a bug in `deleteMarkupBackward` that would cause it to delete pieces
of continued paragraphs below list item markers.

See https://discuss.codemirror.net/t/markdown-input-exception/6133

src/commands.ts

@@ -209,7 +209,11 @@ export const deleteMarkupBackward: StateCommand = ({state, dispatch}) => {
         if (pos - line.from > spaceEnd && !/\S/.test(line.text.slice(spaceEnd, pos - line.from)))
           return {range: EditorSelection.cursor(line.from + spaceEnd),
                   changes: {from: line.from + spaceEnd, to: pos}}
-        if (pos - line.from == spaceEnd) {
+        if (pos - line.from == spaceEnd &&
+            // Only apply this if we're on the line that has the
+            // construct's syntax, or there's only indentation in the
+            // target range
+            (!inner.item || line.from <= inner.item.from || !/\S/.test(line.text.slice(0, inner.to)))) {
           let start = line.from + inner.from
           // Replace a list item marker with blank space
           if (inner.item && inner.node.from < inner.item.from && /\S/.test(line.text.slice(inner.from, inner.to)))

test/test-commands.ts

@@ -190,4 +190,7 @@ describe("deleteMarkupBackward", () => {
 
   it("does noting in a continued list item", () =>
     test("- Foo\n-\n  |Welcome", "- Foo\n-\n  |Welcome"))
+
+  it("doesn't delete normal text in continued list items", () =>
+    test("- \na |b", "- \na |b"))
 })