How do I use the Highlight Selection tool after taking a screenshot?

After taking a screenshot in macOS, I bring up the markup dialog _(introduced in macOS Mojave)_ and try to use the highlighter tool, but it seems to do nothing. Am I just using it wrong? I'd like to add yellow highlighter to some text in my screenshot. ### Steps to reproduce 1. Initiate a screenshot with ⌘ Cmd - ⇧ Shift - 5 1. Ensure that Options -> Show Floating Thumbnail is enabled 1. Capture a still image 1. Double-click the thumbnail that appears in the bottom right corner. This reveals the markup dialog. 1. Press the Highlight Selection pen tool ### Expected Result Dragging my cursor across the image should highlight white areas in yellow ### Actual Result Dragging my cursor across the image moves the entire window without changing the image content