Zooming and letterboxing

Zooming changes reported window size, so letterboxing should be applied. This does not work at the moment or is buggy:

1. does not letterbox on zoom event, only after resizing the window
2. resizing the window on a zoomed tab applies proper letterboxing (multiples of 100) only after resizing the window back to its standard/default size
3. when resizing the window back to its default size, the letterboxing is still applied to all other non-zoomed tabs, making their size smaller than standard despite the available space for default size