"Sketch -> Include Library" menu scales while scrolling

When using the mouse scroll wheel to scroll from the top of the "Sketch -> Include Library" to the bottom, there is one point where the scaling changes.
When in the left situation and scrolling 1 'tick' down, the right situation occurs (where all items take slightly more vertical space):

Reproduce the scaling change by scrolling through the "Sketch -> Include Library" menu in Arduino IDE 1.x, comparing adjacent scroll positions. Then locate the code responsible for that menu's layout and scrolling behavior; done means the item spacing remains consistent throughout the menu.

Written by the indexing model from the issue text.