fix: tabpanel with 'equalalways' issue

[h-east]

When displaying a tabpanel and multiple Windows, 'equalalways' has no effect if you hide the tabpanel.

How to repro.

1. Start Vim with some args.

$ vim --clean --cmd "lang C" +"set stpl=2" +"tabnew aaa" +vsp

2. tabpanel off

set stpl=0

Unexpected behaviour

• The windows are not made the same size after tabpanel disappear.

Expected behaviour

• The windows are made the same size.

[chrisbra]

thanks