I'm repeatedly finding Safari pages cannot scroll up or down in Safari 5.1 on my MacBook Pro. On affected pages, the two finger trackpad gesture has no effect, nor does pressing the up/down keyboard arrows. This behavior happens whether I have tabs open or not. (Right now, I have seven tabs open on one window; pages in three of the tabs refuse to scroll. In another window I have no tabs open; this page also will not scroll.) Reloading pages has no effect.