In View --> Layout, open another column(s). When working in any column other than the first/leftmost column, clicking on the "Open Panel Completions" of a suggestion does not open the panel (it only opens when working the leftmost column).
In multiple rows also revealed odd behavior: with two rows, when working in the lower row, clicking on "Open Panel Completions" changed the layout to 2 columns and did not open the Panel Completions). Working in the top row was fine.
I suppose this is because the panel is the second column(?), but it's quite confusing.
(LSP version 2.3.0, LSP-copilot version 0.3.5, Sublime Text build 4200)