XWIKI-13528: Add support for replacing the default WYSIWYG editor

* We cannot overwrite the existing bindings in the Velocity context when copying script context bindings because we have Velocity scripts that expect $doc instance to be shared among them.

xwiki-platform-core/xwiki-platform-edit/xwiki-platform-edit-api/src/main/java/org/xwiki/edit/AbstractEditor.java

@@ -47,15 +47,23 @@ public abstract class AbstractEditor<D> implements Editor<D>
     private ScriptContextManager scripts;
 
     @Override
+    @SuppressWarnings("unchecked")
     public String render(D data, Map<String, Object> parameters) throws EditException
     {
         ScriptContext scriptContext = this.scripts.getScriptContext();
-        scriptContext.setAttribute(EDIT_CONTEXT_KEY, getEditContext(data, parameters), ScriptContext.ENGINE_SCOPE);
-        try {
-            return render();
-        } finally {
-            scriptContext.removeAttribute(EDIT_CONTEXT_KEY, ScriptContext.ENGINE_SCOPE);
+        // DefaultVelocityManager doesn't overwrite the existing keys when copying the script context bindings so we
+        // have to reuse the existing binding for the edit context.
+        Object editContext = scriptContext.getAttribute(EDIT_CONTEXT_KEY, ScriptContext.ENGINE_SCOPE);
+        Map<String, Object> editContextMap;
+        if (editContext instanceof Map) {
+            editContextMap = (Map<String, Object>) editContext;
+            editContextMap.clear();
+        } else {
+            editContextMap = new HashMap<>();
+            scriptContext.setAttribute(EDIT_CONTEXT_KEY, editContextMap, ScriptContext.ENGINE_SCOPE);
         }
+        editContextMap.putAll(getEditContext(data, parameters));
+        return render();
     }
 
     protected Map<String, Object> getEditContext(D data, Map<String, Object> parameters)

xwiki-platform-core/xwiki-platform-oldcore/src/main/java/com/xpn/xwiki/render/DefaultVelocityManager.java

@@ -168,7 +168,11 @@ private void copyScriptContext(VelocityContext vcontext, ScriptContext scriptCon
         Bindings bindings = scriptContext.getBindings(scope);
         if (bindings != null) {
             for (Map.Entry<String, Object> entry : bindings.entrySet()) {
-                vcontext.put(entry.getKey(), entry.getValue());
+                // Not ideal since it doesn't allow us to modify a binding but it's too dangerous for existing velocity
+                // scripts otherwise (e.g. each velocity script would have a separate instance of $doc).
+                if (!vcontext.containsKey(entry.getKey())) {
+                    vcontext.put(entry.getKey(), entry.getValue());
+                }
             }
         }
     }