Thank you for contacting Syncfusion support.
We have checked your query “Outlining is not shown in EditControl when assign a text to Text property” and we have prepared the sample, how the outline support is enabled using SupportsOutlining property in EditControl. You can collapse/expand ranges of text defined by the language. Please find the code snippet, screenshot and sample for the same.
this.SupportsOutlining = true;
Please check the sample and let us know if you still facing the issue? If not, please modify the sample based on your scenario and revert us with the more details, so that it will be helpful for us to check on it and provide you the solution at the earliest.
Subburaj Pandian V