Basic

adds the scrollbar only if the bottom one is present, does not recompute the top scrollbar

AAAAAAAAA	BBBBBBBBB	CCCCCCCCC	DDDDDDDDD	EEEEEEEEE	FFFFFFFFF	GGGGGGGGG	HHHHHHHHH
AAAAAAAAA	BBBBBBBBB	CCCCCCCCC	DDDDDDDDD	EEEEEEEEE	FFFFFFFFF	GGGGGGGGG	HHHHHHHHH
AAAAAAAAA	BBBBBBBBB	CCCCCCCCC	DDDDDDDDD	EEEEEEEEE	FFFFFFFFF	GGGGGGGGG	HHHHHHHHH

AAAAAAAAA	BBBBBBBBB	CCCCCCCCC	DDDDDDDDD	EEEEEEEEE	FFFFFFFFF	GGGGGGGGG	HHHHHHHHH
AAAAAAAAA	BBBBBBBBB	CCCCCCCCC	DDDDDDDDD	EEEEEEEEE	FFFFFFFFF	GGGGGGGGG	HHHHHHHHH
AAAAAAAAA	BBBBBBBBB	CCCCCCCCC	DDDDDDDDD	EEEEEEEEE	FFFFFFFFF	GGGGGGGGG	HHHHHHHHH

Recompute on window resize

adds the scrollbar only if the bottom one is present, and recompute the top scrollbar size each time the window is resized

resize the window to try!!!!

AAAAAAAAA	BBBBBBBBB	CCCCCCCCC	DDDDDDDDD	EEEEEEEEE	FFFFFFFFF	GGGGGGGGG	HHHHHHHHH	IIIIIIIII	JJJJJJJJJ
AAAAAAAAA	BBBBBBBBB	CCCCCCCCC	DDDDDDDDD	EEEEEEEEE	FFFFFFFFF	GGGGGGGGG	HHHHHHHHH	IIIIIIIII	JJJJJJJJJ
AAAAAAAAA	BBBBBBBBB	CCCCCCCCC	DDDDDDDDD	EEEEEEEEE	FFFFFFFFF	GGGGGGGGG	HHHHHHHHH	IIIIIIIII	JJJJJJJJJ