Version:           3.2.1 (using KDE 3.2.1, compiled sources)
Compiler:          gcc version 3.2
OS:          Linux (i686) release 2.4.24-1up.1

Generally, with KDE scrollbars, middle mouse button drag anywhere in the scrollbar area means the following:

+ Mouse-down moves the thumb to the clicked location.
+ Drag moves the thumb, as with left-button drags.

This behavior is consistent across all apps and widgets, except for Konqueror views.  In 3.2.1, middle mouse button on Konqueror scrollbars does the following:

+ Mouse-down does nothing.
+ Mouse-up moves the thumb to the clicked location.
+ Mouse-down, followed by drag opens a new window (!)

I'm pretty sure this is a bug.  This is consistent across different Konqueror view types (happens with HTML part, file view, or embedded KPDF part, for example).  Also pretty sure that it's a Konqueror problem: other embedded HTML parts, for example (such as KMail's message view) do not have this behavior.

Possibly related, I am also seeing general weirdness with Konqueror being too eager to grab the middle mouse button.  Try the following:

1. Set kwin mouse policy to "focus follows mouse".
2. Open a Konqueror window on some web page.
3. Mouse over to Kicker, but make sure the Konqueror window remains the "active" window (i.e., the title bar does not go grey).
4. Middle-button drag some of your application buttons (which should rearrange them).

The above does not work for me.  Instead, Konqueror opens a new window or tab.  Konqueror seems to grab middle mouse clicks onto empty desktop area too.  Oddly, left mouse button click at step 4 works fine (launches an app for Kicker, or gives focus to the desktop).