Snags whats on the screen but that's it.... I tried it as all-in-one and under image in scrolling window. Works fine in Version 13.
in Snagit 13, when Selection was set to Window, capturing provided an option to capture the selected region, capture a scroll, resize, redo, cancel etc. Its gone in 2018. I vote to bring that feature back because it allowed an easy, in place choice of what to capture. Its absence in 2018 means more keystrokes, more effort and slower workflow.
Apparently this is a known issue. Here's a link to a TechSmith article. In addition to the two workaround options provided, setting Snagit 2018 to All In One lets you select a scrolling window capture after selecting the region to be captured. And that option works ok in Firefox.