If you do a build or update from version control or anything else that has  
its own results panel that will take its place, but if you click on the  
find button below the results area it will bring back the panel with the  
results from your last search.

Dan.

Look at the three buttons at the right of the toolwindow's titlebar.

The rightmost is minimize - you know what it does.

The one with the pushpin is pin/unpin. When a window is unpinned, it
disappears when it loses focus. Convenient for short-term tasks.

The button with one rectangle is dock/undock. When a toolwindow window
is docked it replaces the previous window and pushes it on the "replaced
toolwindow" stack. When a toolwindow is undocked it just slides over the
previous toolwindow and slides out when you change focus (there is
subtle difference from docked+unpinned).

The leftmost is 'pop out'. Often you'd want to move a toolwindow  on the
second screen.

For more details and pictures, go to IDEA's help and look under > User
Interface > Tool Windows.