This bug / problem has been introduced with JavaFX 8u40, JavaFX up to 8u31 works fine. Would be great when the final 8u40 can be fixed.

I'm using the percentage layout in a GridPane to ensure that all grid cells have the same width and height (ColumnConstraints.setPercentWidth()). So I'm able to setup the layout of a custom calendar component with a monthly view.
When resizing the window and so the GridPane then the cell size must resize automatically and must use all the available space of the GridPane.

This worked fine, but since JavaFX 8u40 there is a bug - the cells don't use all the available width anymore. There's mostly empty space after the last column, this will be filled sometimes when the user increases the window width. The height layout and resizing works as expected.
Perhaps this problem is caused by a rounding issue in the width computation.