How to avoid moving everything in the page down when submenu shows up

thanks for your input adam, it definitely is something but sadly enough I cannot make it work on the site I am building.

My knowledge of css is quite limited (I am actually learning on my own as I develop the site), but it seems to me you are not using the menu within a block, am I right?
Also, are you using your navigation menu? I am using a custom one instead, but even when I changed it to the navigation menu it did nothing. The images were not loaded in any case, and I did get no path errors in the reports either. Should I change your strings
#navigation_menu
to
#my-custom-menu_menu
to make it apply to my menu?

UPDATE:
I managed to avoid the annoying vertical movement with the trick shown in http://drupal.org/node/246655 by adding this to my css (still no luck with theming the menu links though):

#block-dynamic_persistent_menu-0
{
  height:3em;
}

no worries, I was playing around and got most of it working. What I am struggling with at the moment is changing the looks of:
- main menu links without submenus do not show as active when the page they point to is active
- if a submenu item is visited, its parent is not shown as active

Here's the code I am using. Background images, which do not change, are actually in the header background image:

/* dynamic consistent menu */
#block-dynamic_persistent_menu-0 {
	line-height:25px;
	height:4em;
	text-transform:uppercase;
	margin:10px;
}
.dynamic-persistent-menu-menu {
	height:3em;
	padding: 2em;
}
.dynamic-persistent-menu-menu-item a {
	/*---background-color:#444444;---*/
	padding-left: 6px;
	padding-right: 10px;
	font-size:1em;
	font-weight:600;
	text-decoration:none;
	color:#cccccc;
	margin: 6px;
	border-width:1px;
	border-left-color:#414141;
	border-left-style:solid;
	}
.dynamic-persistent-menu-menu-item a:hover {
	color:#ffffff;
	border-left-color:#404040;
}

.dynamic-persistent-menu-menu-item a.active, .dynamic-persistent-menu-menu-item a.active-trail {
color:#c8d636;
}
.dynamic-persistent-menu-sub-menu {
	height:3em;
	/*---background-color:#777777;---*/
	margin:0;
}

.dynamic-persistent-menu-sub-menu-item a {
	padding-left: 6px;
	padding-right: 10px;
	font-size:1em;
	text-decoration:none;
	color:#717171;
	margin: 5px;
	border-width:1px;
	border-left-color:#cccccc;
	border-left-style:solid;
}

I am attaching a few images showing the current behaviour (my site is not quite visible yet, sorry). You can see the current page title in the middle left hand side of the image, the title of the picture also tries to describe the behaviour.

Any ideas why the items in pictures 03xxx and 04xxx are not highlighted?

Thanks a lot for your help!

PS I am actually using Firebug, but I must admit I am a bit overwhelmed by the sheer amount of information it provides ;)

Strange. In the example provided for the module you can see the exact behaviour I was writing about. See:
http://www.e-joussour.net/en/category/7/156/171 where both submenu and parent are active, unlike in my 03_xx picture
and
http://www.e-joussour.net/en/category/7/194 where top menu (without submenus) is active, just what I would like to happen in my 04_xx picture). Am I using the .active class wrong here? should it be a:active instead?

It feels better that this is not such a simple issue to deal with, though, I will keep on trying and let you know if I find anything. In the meantime any feedback which may cast some light on the matter is much welcome.

PS
when I manage to get this working I will write a short howto/documentation with a list of the css classes and the items they affect for this fantastic menu.

Your very determined.

Yep, but do let me know if I am getting a bit of a pain... I am perfectly aware that this issue has derived far far away from its original point, which is, btw, already solved. Perhaps I should start a new issue and mark this one as closed. But I am not familiar with the issue tracking system and would rather let the maintainer choose what to do with this.

Thanks for sharing that site

I deserve no credit for this, thank the module's author instead as this is the official demonstration for dpm.

Your ramblings are most welcome, and make a lot of sense to me. If only I had the skills to turn that into php code... I tried yours instead (added it to the end of my theme's template.php), but nothing changed at all.
I also checked the template.php file in garland and could not find any references to the trail class at all.

Thanks a million for your time and patiente, adam. And do let me know if this has already gone too far and you do not wish to waste any morer time on it. No offence will be taken.

it does, thanks a lot Branjawn!!