FIX: Don't set `categoryId` on PM drafts (#20475)

app/assets/javascripts/discourse/app/models/composer.js

@@ -142,7 +142,9 @@ const Composer = RestModel.extend({
     set(categoryId) {
       const oldCategoryId = this._categoryId;
 
-      if (isEmpty(categoryId)) {
+      if (this.privateMessage) {
+        categoryId = null;
+      } else if (isEmpty(categoryId)) {
         // Check if there is a default composer category to set
         const defaultComposerCategoryId = parseInt(
           this.siteSettings.default_composer_category,

app/assets/javascripts/discourse/tests/unit/models/composer-test.js

@@ -317,6 +317,20 @@ module("Unit | Model | composer", function (hooks) {
     );
   });
 
+  test("initial category when creating PM and there is a default composer category", function (assert) {
+    this.siteSettings.default_composer_category = 2;
+    const composer = openComposer.call(this, {
+      action: PRIVATE_MESSAGE,
+      draftKey: "abcd",
+      draftSequence: 1,
+    });
+    assert.strictEqual(
+      composer.categoryId,
+      null,
+      "it doesn't save the category"
+    );
+  });
+
   test("open with a quote", function (assert) {
     const quote =
       '[quote="neil, post:5, topic:413"]\nSimmer down you two.\n[/quote]';