Letting GTK+ automatically manage the size of the brush preview frame may cause undesired behaviour caused by GTK+ trying to evenly divide the available space between the frame and the options box. Clicking a brush may lead to a change in the size of the frame when the content of the options box changes. This is especially annoying when the number of columns in which the brushes are displayed changes and the brush selected by the user jumps away.
Fix: in brush_select_new() of brush_select.c, change
278 gtk_box_pack_start (GTK_BOX (hbox), bsp->frame, TRUE, TRUE, 0);
278 gtk_box_pack_start (GTK_BOX (hbox), bsp->frame, FALSE, FALSE, 0);
In addition, some manual management of the size of the frame in question would seem appropriate (e.g., depending on the size of the window).