Thanks,To Dock the Slide Panel again, grab the Slide Panel window the closest possible to the top left corner (just before the cursor changes to a resize cursor) Then drag the panel slowly to the place where it was docked before (just above the Normal tab). At some point you should see the gray contour showing up. Release the mouse and it should be docked.
However, I have to stress that: We need to drag the panel very very slowly to the left corner
When the panel and the edge of LibreOffice's main window becomes close (the distance is
as short as 1 or 2 pixels), we release the mouse.
It worked, but not so friendly, my wish is
1) Double click makes the panel docked
2) Make it easier to drag the panel
(Like LibreOffice magetic to stick the panel :))
Nguyen Vu Hung