Can reproduce on current Git geany on Linux ... and with Xed, and with Gedit and any other GTK app I tried. The narrowing happens when the mouse cursor is moved off the scroll handle a small amount while holding it (left mouse depressed), then the speed reduces.
So its a GTK thing, but is it deliberate?
From [this](https://blog.gtk.org/2017/10/11/a-scrolling-primer/) looks like it is, my emphasis:
"There’s more to scrolling in GTK+ that you may not know about. One feature that we introduced long ago is a ‘zoom’ or ‘fine adjustment’ mode, which slows the scrolling down to allow pixel-precise positioning. To trigger this mode you can either use a __long press__ or shift-click in the slider. As you can see in the video, once you move the pointer below or above the scrollbar, it will keep scrolling at the same relaxed speed until you let go."