Hi there!
Is it possible to set a boundry to the display of a tile layer by a polygon?
I think you are asking if it is possible to display a part of a tile layer that intersects with a given polygon geometry. If that is the case, then yes you can with the use of destination-in effect. Please take a look at the this sample as it shows what you are asking for: https://developers.arcgis.com/javascript/latest/sample-code/effect-blur-shadow/
https://developers.arcgis.com/javascript/latest/sample-code/effect-blur-shadow/
