Nightly build version WebKit-SVN-r18821.dmg (I would've thought this is 420+ (nightly), but the About box reports 419.3; nevertheless, Finder Info reports 420+). Pressing spacebar with focus in an edit field causes page to scroll. 1. Find a page with a form, long enough to have a vertical scroll bar. (YouTube works; there's a search form at the top). 2. Click in any text field (I did not try with password field). 3. Start typing, press spacebar. 4. Note page scrolls. Next character (even space) will try to scroll field back into view.
I think I accidentally built the r18821 nightly with a work-in-progress patch in my tree. It's related to scrolling with the keyboard, which would explain this perfectly. I can't reproduce this with the latest nightly build (r18836). Sorry for the inconvenience.
This version doesn't have the bug: WebKit-SVN-r18836.dmg Thanks!