While the bug will be triggered in this scenario, I found that it was not necessary to switch back to the source editor to trigger it. But it is necessary to "publish" the edit to observe that all other text has been erased from the page.

Also I should mention that the "Hide" and "Show" links are diametrically opposed, on the far right and far left side of the page, respectively, so toggling them makes for laborious, wide mouse movements and a bad UX, and toggle them is exactly what we must do every time we need to toggle "Live Updates"...

I am the editor who opened bug T329422. I noticed that simply wanting to hide excess controls on the page also hid an important and, most importantly, dynamic control that resets every time the page reloads! This is a bad UX if we can't even tell the status of "Live Updates" because it is hidden behind an unrelated cluster of controls. The "View new changes since..." control is also dynamic, and of course related to this fundamental "Live Updates" button.