This commit refines bidi property handling:
* experimental_bidi has been split into two new configuration settings;
`bidi_enabled` (which controls whether the terminal performs implicit
bidi processing) and `bidi_direction` which specifies the base
direction and whether auto detection is enabled.
* The `Line` type can now store those bidi properties (they are actually
split across 3 bits representing enabled, auto-detection and
direction)
* The terminal now has a concept of active bidi properties and default
bidi properties
* The default properties are pulled from the wezterm configuration
* active bidi properties are potentially set via escape sequences,
BDSM (which sets bidi_enabled) and SCP (which sets bidi_direction).
We don't support the 2501 temporary dec private mode suggested by
the BIDI recommendation doc at this time.
* When creating new `Line`'s or clearing from the start of a `Line`, the
effective bidi properties are computed (from the active props,
falling back to default propr) and applied to the `Line`.
* When rendering the line, we now look at its bidi properties instead
of just the global config.
The default bidi properties are `bidi_enabled: false` and
`bidi_direction: LeftToRight` which corresponds to the typical
bidi-unaware mode of most terminals.
It is possible to live reload the config to change the effective
defaults, but note that they apply, by design, to new lines being
processed through the terminal. That means existing output is
left unaffected by a config reload, but subsequently printed lines
will respect it. Pressing CTRL-L or otherwise contriving to have
the running application refresh its display should cause the
refreshed display to update and apply the new bidi mode.
refs: #784
In order to support RTL/BIDI, wezterm needs a bidi implementation. I
don't think a well-conforming rust implementation exists today; what I
found were implementations that didn't pass 100% of the conformance
tests.
So I decided to port "bidiref", the reference implementation of the UBA
described in http://unicode.org/reports/tr9/ to Rust.
This implementation focuses on conformance: no special measures have
been taken to optimize it so far, with my focus having been to ensure
that all of the approx 780,000 test cases in the unicode data for
unicode 14 pass. Having the tests passing 100% allows for making
performance improvements with confidence in the future.
The API isn't completely designed/fully baked. Until I get to hooking
it up to wezterm's shaper, I'm not 100% sure exactly what I'll need.
There's a good discussion on API in
https://github.com/open-i18n/rust-unic/issues/273 that suggests omitting
"legacy" operations such as reordering. I suspect that wezterm may
actually need that function to support monospace text layout in some
terminal scenarios, but regardless: reordering is part of the
conformance test suite so it remains a part of the API.
That said: the API does model the major operations as separate
phases, so you should be able to pay for just what you use:
* Resolving the embedding levels from a paragraph
* Returning paragraph runs of those levels (and their directions)
* Returning the whitespace-level-reset runs for a line-slice within the
paragraph
* Returning the reordered indices + levels for a line-slice within the
paragraph.
refs: https://github.com/wez/wezterm/issues/784
refs: https://github.com/kas-gui/kas-text/issues/20
