Most other outline apps have a search box with the ability to star, bookmark or otherwise save a search to be recalled later, effectively creating a saved filter. A lot of users have asked for the ability to save a filter in Legend, which is currently best accomplished by using mulitple panes. So given that, how about this:
Add a “Save as Pane Default” button to the filter panel, so that when you click the X, the filter settings revert back to that saved default. The initial default for every new pane would be to hide complete ( -is:complete ).
This way users could use the filter box to temporarily make multiple changes to the filter or for searching (which I still do because it works like search in other outliners, and ctrl-F and shift-ctrl-F don’t work for me plus a lot of users will never discover these hidden commands) and then quickly revert back to the default filter. Right now, it seems that there is an initial pane filter state that is saved when you switch boards or exit the app, and recalled when you return to the board. While you can revert back to this filter state by using the Back button in the pane header, if you switch boards or exit the app without clicking all the way back to that initial filter in each pane first, then you may lose that filter state since the current state will become the new initial state, with no way to recover that initial filter state since the back history is cleared when changing boards or exiting the app.
So this would require you to save not only the current filter state (to be restored upon returning to a board, as you do now) but an additional default state (to be restored when a user clears the filter). When restoring a pane with current state different than the default state, you could also set the filter to the current state and make that the second state in history, with the first history state set to the default so you could navigate back to it with the Back button (or save the entire history).