It's the text input field at the bottom. It's active the moment the page loads and anytime you hit PgUp, PgDn, or the arrow keys it cause the page to jump all the way down until the cursor is visible in the field.
I have no idea how to circumvent that except to just click on any empty space on the thread to deactivate the input field.
I have no idea how to circumvent that except to just click on any empty space on the thread to deactivate the input field.