horizontal scrollbar under the thumbnail window is missing part of the scroll bar and harder to grab

Did you mean it has shrunk in height, or in width?

I see no difference in height, and I believe it has always been the case that this scrolling handle would vary in its width, according to what proportion of the overall number of images your display is able to show.

The screenshot suggests that your filmstrip included about 600 images at that moment. When more images are present it will show narrower still (as a natural preview, of what scrolling opportunity you have got)

whereas when your filmstrip includes only a few more thumbnails than can be shown at the same time, that bar will nearly fill the width of the display accordingly.

Building on Califdan’s reply, a full-resolution screenshot of the entire LR window might help.

You mean the scroll bar under the Filmstrip.  It looks ok in your screen shot.  What is the problem with it?