The bug report explains how to reproduce the problem (basically, Ctrl+F two times in succession).
The bug itself comes about in a very roundabout manner (explained in the commit message), but ultimately is caused by EmulatedCommandBar::SwitchToMode calling SearchMode::deactivate even when the newMode is also SearchMode, meaning the search bar is hidden every time. The only change is that EmulatedCommandBar::SwitchToMode just returns and does nothing when newMode == m_currentMode since most of the function is just setting pointers/members.
BUG 368130