1. In normal text editing mode. (Not the new Cell Selection mode)
2. Turn off text wrapping if not already off. (i.e. Ctrl+1)
3. Make a box selection by holding the Alt key and dragging the mouse cursor across multiple lines of text.
4. With the box of text highlighted (selected), press the either the Left or Right key on the keyboard.
5. Then press the ESC key. This leaves a non-highlighting text cursor that spans multiple rows.
6. Next, press the End key, or hold down on the Right key, to take the multi-row-spanning cursor to the end of the line.
Supposing the editor window is smaller in width than the width of the line, performing step six is expected to induce horizontal scrolling so that the text cursor is always visible on the screen.
As the cursor moves right in step six, it moves off bounds of the editor screen and becomes hidden. The window does not automatically scroll horizontally.
I’m fairly sure this worked as expected prior to 15.8.0.