* Calculate tab dimensions more correctly. (regression on 4.0.11)
* Fix ability to override tab size with `--tab-size`.
* Use `ButtonText` instead of `ButtonShadow` as the tabs border color in the "High Contrast" theme, for better visibility.
* Don't apply new-tab-command-tab behavior for non-new-tab-command-tabs accidentally, e.g. tabs opened by other applications.