In the reader, clicking the jump-chapter drop-down menu and cancelling that (click anywhere else) will make the next arrow key stroke change chapter

[Speedphoenix]

In the reader, click the chapter drop-down menu

id="jump-chapter"

, but don't choose any of the chapters from the drop down and cancel that drop down (click elsewhere for example)
In that state, the next action may do different things:
[ol]A mouse click will behave normally (and cancel that state, even if clicked empty space, so long as it's inside the reader's

div

)
Keyboard shortcuts are disabled (including wasd)
Pressing the arrow keys UP or LEFT arrow will make you jump to the NEXT chapter
Pressing the arrow keys DOWN or RIGHT arrow will make you jump to the PREVIOUS chapter, first page[/ol]
Note that the direction of the chapter jump is the same regardless of the direction in the reader settings

This happens on both Chromium and Firefox, running on Ubuntu 18.04

Edit: this was actually just the blue outline when the drop down is selected not appearing on the Dark-Bronze site theme
The outline is not there on the Dark-Bronze, Light-Slate, Dark-Slate and Abyss site themes