Comment 33 for bug 107247

Repeated tests to try to reproduce it (each started from a fresh page load):

1. Yes: PageDn (x3), PageUp (x3), page "locked", caret there
2. Yes: PageDn (hold), PageUp (hold), page locked, click for caret to appear
3. No.
4. Yes: PageUp (repeat), PageDn (repeat), PageUp (repeat), PageDown (repeat), Arrow Down, caret on
5. Yes: Hold Down Arrow whilst page loading, Page jumps back (almost) to top, page "locked", click for caret to show
6. Yes: Hold Down Arrow whilst page loading, scroll down, jump back to top, scolls down again, jump to top again, page locks; click to different tag, click back again, page unlocks.
7. Yes: Page locked itself, click for caret to appear, change tabs and back again, page unlocked.
8. Yes: Hold Arrow Down whilst page loading, page jumped + locked, change tabs and back to unlock.
9. Yes: Hold Up Arrow whilst page loading, page locked
10. Yes: Hold Page Down whilst page loading, page jumped to top and locked, click for caret to appear.

So, nine times out of ten, reproducibility. The most reliable way (although not the only way) appears to be to hold down one of the scrolling (Arrow or Paging) keys whilst the page is loading. And that changes tabs clears caret browsing mode.

The Downstream bug report has that produce cross-platform (and cross browser) so appears to be Gecko itself kicking into caret mode, rather than the browser interface enabling it.