Automatically collapse empty lists setting
From Planfix
The setting Automatically collapse empty lists in Planner list settings controls the appearance of a list depending on the objects in it (tasks or contacts):
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If this setting is enabled, then when there are no tasks in a list, the list will automatically be collapsed into a vertical or horizontal panel (depending on its location in the planner):
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If this setting is not enabled (by default it isn't) its appearance will not depend on whether or not there are objects in it.
Useful information
To expand an automatically collapsed list, simply click the panel it collapsed into