Ctrl+Shift+U ran the Ctrl+U shortcut on Windows and Linux and underlined
the selection. In Excel it expands or collapses the formula bar, on
Windows and as Control+Shift+U on a Mac. It now does that on the grid
and in the cell editor or formula bar while editing, and the entry stays
open.
The new UI command view.expandFormulaBar {on?} sets the state, or
toggles it without a parameter, and returns {on}. The chevron at the end
of the formula bar is unchanged.