don't put cursor position in NewlinePart after adding it

You can't append to it anyway, so mark it uneditable and skip
uneditable parts if that's where an edit ended up.

This has the added advantage that if there is text after a newly
insert pill, the cursor will be put just before it rather than
in the pill, after the last character.

src/editor/model.js

@@ -88,7 +88,8 @@ export default class EditorModel {
         }
         this._mergeAdjacentParts();
         const caretOffset = diff.at - removedOffsetDecrease + addedLen;
-        const newPosition = this._positionForOffset(caretOffset, true);
+        let newPosition = this._positionForOffset(caretOffset, true);
+        newPosition = newPosition.skipUneditableParts(this._parts);
         this._setActivePart(newPosition);
         this._updateCallback(newPosition);
     }
@@ -261,4 +262,13 @@ class DocumentPosition {
     get offset() {
         return this._offset;
     }
+
+    skipUneditableParts(parts) {
+        const part = parts[this.index];
+        if (part && !part.canEdit) {
+            return new DocumentPosition(this.index + 1, 0);
+        } else {
+            return this;
+        }
+    }
 }

src/editor/parts.js

@@ -205,6 +205,14 @@ export class NewlinePart extends BasePart {
     get type() {
         return "newline";
     }
+
+    // this makes the cursor skip this part when it is inserted
+    // rather than trying to append to it, which is what we want.
+    // As a newline can also be only one character, it makes sense
+    // as it can only be one character long. This caused #9741.
+    get canEdit() {
+        return false;
+    }
 }
 
 export class RoomPillPart extends PillPart {