This is from a recent support case with a customer. The HTMLFieldControl for the Description and Repro Steps fields on a modified WI are not expanding in IE 11 and IS expanding in Chrome.
This is a known issue and is fixed in TFS 2015 CU3. The work-around is to run IE11 in compatibility mode for the intranet site. Open IE, go to Tools -> Compatibility View Settings and uncheck the checkbox which says “Display intranet sites in Compatibility view” and this should fix this issue.