This feature is currently only implemented in Outline and Gaudi views.
The view filter toolbar is hidden by default, but can be opened from the View menu, or via a shortcut.
Filter expressions are queries, and are saved with the tab.
When the filter bar is active, outlines do not draw collapse widgets and all containers are implicitly expanded. Closing the filter bar disables the filter and restores the previous expansion state
The filter list is alphabetised.
The filter bar displays the number of items that match the filter's query. Note that this is not identical to the number of items displayed in the outline, since an item appears in a filtered outline if it matches the query or if one of its descendants matches the query.
The Query field of the filter bar grows larger if the window is sufficiently wide.
See also—notes linking to here: