Section collapses when selecting request header text to copy

**Link to problem area**: https://spec.matrix.org/

**Issue**
When selecting the request path to copy, the section collapses when releasing mouse button.

https://user-images.githubusercontent.com/19155609/131351816-19b4d5ad-9551-4709-a084-79cd2d41a678.mov

Start by reproducing the selection behavior on the request path at spec.matrix.org using the linked video as a reference. Trace the page's section-collapse behavior and identify the code handling mouse selection; done means copying selected request-header text no longer collapses its section.

Written by the indexing model from the issue text.