CSS: Fix default theme sidebar on wide, short windows.

[edunham]

In a tall, wide window, the sidebar sits at the left and all entries are visible. This is good.

In any narrow window, sidebar entries (other pages on site) flow to the top of the page, and all are visible. This is good.

But if a window wide enough for the menu to stay at the left gets shrunk vertically to half the height of the screen or so, only part of the sidebar is visible. If the site has several pages, those at the bottom of the list are not displayed. There appears to be no way to force links to pages at the end of the list to show up -- mousing over the sidebar and moving the scroll wheel doesn't move them either. This makes Tinkerer sites hard to use for people who can't, don't want to, or don't know to resize their window narrow enough to force the sidebar entries to flow to the top.

Here's my Tinkerer site in a normal window (top) compared to a wide-but-short window (bottom):

Correct behavior would be anything that allows a user to access an arbitrarily long list of pages in the sidebar in a wide-but-short window. The simplest thing to do would probably be to make it so that scrolling on the list causes it to move, but I don't currently have sufficient web dev knowledge to implement that.