width calculations are broken with vertical tabs

[sh-dave]

When using vertical tabs, the remaining window space is calculated wrong. It seems it's not subtracting the width of the tabs area. In the screenshot, the marked lines are comboboxes that extend further than the window width. As a workaround i'm using manually adjusted rows, thatswhy the commit-comboboxes below each broken one look ok.

z.row([4 / 5, 1 / 5]);
    z.combo(...);
    z.text('');