* fix: check if tabline is present when calculating certain layouts
- also tweaks height calculation for `center` strategy
* refactor: remove `center` strategy tweak
- will reimplement something similar in another PR for a few of the strategies
* fix: check if `preview.line` is `nil`
* refactor: factor out `tbln` calculation and `max_lines` adjustment
- also tweaked `vertical` calculations so that no `nil` check is required
d057b10503