Properly detect changes that should reset cantEdit flag

Issue #5974

Author: marijnh

src/model/Doc.js

@@ -75,7 +75,6 @@ Doc.prototype = createObj(BranchChunk.prototype, {
     let top = Pos(this.first, 0), last = this.first + this.size - 1
     makeChange(this, {from: top, to: Pos(last, getLine(this, last).text.length),
                       text: this.splitLines(code), origin: "setValue", full: true}, true)
-    this.cantEdit = false
     if (this.cm) scrollToCoords(this.cm, 0, 0)
     setSelection(this, simpleSelection(top), sel_dontScroll)
   }),

src/model/changes.js

@@ -202,6 +202,9 @@ function makeChangeSingleDoc(doc, change, selAfter, spans) {
   if (doc.cm) makeChangeSingleDocInEditor(doc.cm, change, spans)
   else updateDoc(doc, change, spans)
   setSelectionNoUndo(doc, selAfter, sel_dontScroll)
+
+  if (doc.cantEdit && skipAtomic(doc, Pos(doc.firstLine(), 0)))
+    doc.cantEdit = false
 }
 
 // Handle the interaction of a change to a document with the editor