Why are my scrollbars going to extremes when pressing their buttons?

With Unity version 2017.3, I am using GUI.VerticalScrollbar and GUI.HorizontalScrollbar in my custom editor (actually, a custom PropertyDrawer; there is a good reason I’m not using utils like GUI.BeginScrollView). However, when I use my scrollbars like this…

EditorGUI.BeginDisabledGroup(maxX == 0);
scrollX = (uint)GUI.HorizontalScrollbar(new Rect(xyPos + new Vector2(slHeight, availableWidth - slHeight), new Vector2(availableWidth - slHeight, slHeight)), scrollX, 1, 0, maxX + 1, GUI.skin.horizontalScrollbar);
EditorGUI.EndDisabledGroup();
EditorGUI.BeginDisabledGroup(maxY == 0);
scrollY = (uint)GUI.VerticalScrollbar(new Rect(xyPos, new Vector2(slHeight, availableWidth - slHeight)), scrollY, 1, maxY + 1, 0, GUI.skin.verticalScrollbar);
EditorGUI.EndDisabledGroup();

What I get is that, if I click either of the < > (for horizontal scrollbar) or ^ V (for vertical scrollbar), the scroll position goes to 0 or maxX respectively for horizontal scrollbar, and maxY or 0 respectively for vertical scrollbar. I don’t get how to just move… by one, when clicking.

After adding some calls to Debug.LogFormat I notice the movement from any position to any of those extreme positions is instantaneous. On the other hand, if I drag the scrollbar thumbs, I can go to the position I want. But this doesn’t happen if I want to click any of the buttons: they move the thumb always to the extreme positions of the scrollbar.

What am I missing here? I’d like to use the scrollbar buttons as I’m used to: pressing them moves the thumb gradually in the appropriate direction, and not extremely like my case.

Found it: Scrollbars don’t go to extremes. In my examples, they went because the scrolling size was less than 10 on either direction. By default scrollbars move by 10 when the buttons are pressed (Unity 2017.3, default skin).