THIS IS AN IDEA, NOT YET PROGRAMMED! The download is simply a picture.
Please have a look at the KDE Wiki at http://wiki.kde.org/tiki-index.php?page=KDE%20Context-Sidebar&comzone=show to participate in further discussion on this.
This is not only about whether one likes it or not, but more about whether new/unexperienced users would have easier access to KDE's functionalities.
Sidebars are/could be a very useful feature, as they allow easy access to information about the filesystem, bookmarks, addressbook you name it.
At the moment haowever they are too static and do not really interact with their environment. Best example is konqueror. Open it, go to a website, hit F9 to enable the sidebar, what do you get, the default setting (mostly filesystem I guess) instead of the sidebar noticing that it is in the context of the internet and should display bookmarks as the most likely. I know you can set this by enabling the sidebar, pick the bookmarks-tab and save the profile. So the user has to know all this and do work to configure what should be obvious and straight forward.
Context-sidebars would do the following:
They are not static and do only appear when they are of use. If so they glue themselves to the side of an app-window so that you do not have a static sidebar at the side of the screen but always next to the app that it is useful for. In that way they are partly like toolbars with text, placed at the right side of an app. However their functionality should go beyond this. In XP Microsoft got this for the filesystem, i.e. you open a folder and the sidebar offers you functionality for the selected files, i.e. copy to.. as te most simple example. Of course one could say that most of these functionalities are already there, context-menu, toolbar and menus, however my experience with most normal users tells me that context-menu, short-cuts, menus and toolbars are for advanced users, as they are not obvious enough in propagating their funtionality. It is not even straigth forward for most new users to simply hover the icons in the toolbar to see their funtionality. Further they do not know that they can enable text next to the toolbar-icons and, as this is useless at the top (too wide), place the toolbar at the side.
So what do I want? I think that definitely for the normal user and also for the advanced user, though in other areas and only if configurable, context-sidebar should offer information and functionality that is otherwise hidden in context-menus or icon-only-toolbars. They should be the obvious link to apps and functionality meant for the selected files or environment.
I guess examples are the best way to show what I mean:
Examples in kmail:
You open/click on an email, the context-sidebar displays (all) available information about the sender so you can answer the email either by calling, IM, a letter or even by email ;), without having to look this information up in the adressbook. For CustomerRelationsManagement there could also be some information about the client, his/her last emails, documents...
You compose an email, the addressbook is next to the composing window, useless if you need only a few recipients in the TO-field, but very useful if you want to add some to TO, some to BCC,CC, some to IM, some to SMS, some to fax. Simply select them and click on the button representing the address-type you want to use. If recipients are chosen, activate the field in the composing window.
At the moment you would have to open the pick-address dialogue, pick the people, close it and then activate the BCC, or CC-field manually. The latter can also be set to permanently, I know, but this wastes a lot of space if you do not need them. On the other hand at the moment these fields are not even activated automatically if the email has recipients in these fields.
You might argue that space is limited for an efficient address-list, thus when clicking into the list it should expand so you can work with it and when clicking back on the email-text become small again automatically. If you need it, it's big, if you do not, you can see it but it does not disturb you. And even if you do not like a list at this point, put a button Adressing there instead of that ... behind the TO-field.
Examples from the filesystem:
You have selected music-files, or are in a directory that consists of only music files, the context-sidebar provides access to a media-player similar to amarok's small window, i.e. more functionality than the sidebar-media-player in 3.1.x had, there is a possibility to add the selected files or the whole directory to the library, or playlist, one can burn songs, or simply display information about the artist. So if you click a button it opens a window with the app the button belongs to, i.e. as if you would chose it from the context-menu and chose add to playlist...
You open a CD/floppy in konqueror, if it's RW/floppy the sidebar provides access to erasing/formating it, a mini-burning-list to drag&drop files from different directories to and burn them, if it's an audio CD it offers KsCDs functionality, or to rip it, if it's a video-CD to play it, rip it...
If you select an cd-image-file the context-sidebar gives you information about the image, the possibility to mount it (there is a script at kde-look.org that can do this) or to burn it.
Selecting an image gives you access to its properties, to start editing it, rotating it, burning them to get them to a photo-shop, connect to you camera, make a slide-show...
Addressbook's context-sidebar would offer the possibility to call, chat, arrange a meeting with..., write an email to...
Internet's context-sidebar should offer bookmarks as well as a button to add, edit hem, a button to start an IM-app, or to start an email-app, online-banking-app, IP-telephony...
So apps would not have to program their own functions to burn a playlist with k3b or rip that cd but simply use an interface to the sidebar, passing on needed information to do the task. As screens are getting bigger, icon view is standard for KDE and most new users and some notebooks even have over-wide screens, I think that space would not be an argument against this, especially not, as the sidebars can be turned off, all-in-all, or per app/context.
The great benefit would be to have functionality and information, available for the eyes of every user without an extra click.
If I am not the only one that is interested in this feature I would setup a site to put more screenshots on. This way ideas about functionality and look could be discussed before it is coded.