Add --allow-fallback option to the map command that controls shifted
and ascii (alternate_key) fallback for individual key mappings.
For non-Latin keyboard layouts, when the current layout key is
non-ascii (codepoint > 127 and < 0xE000), the alternate_key from
the base layout is used for matching if the mapping opts in via
--allow-fallback=shifted,ascii.
Default kitty bindings use --allow-fallback=shifted,ascii so they
work out of the box with non-Latin layouts. User custom mappings
default to --allow-fallback=shifted (preserving existing shifted_key
behavior without ascii fallback).
--allow-fallback=none disables all fallback for a mapping.
Python side: parse_options_for_map() in options/utils.py handles flag
parsing, ShortcutMapping uses it in __init__. get_shortcut() filters
candidates by per-mapping allow_fallback.
Go side: ParseMap() handles --allow-fallback, KeyAction stores
AllowFallback, ShortcutTracker.Match passes it to matching.
MatchesParsedShortcut defaults to shifted,ascii for hardcoded shortcuts.
Migrated kittens (themes, command_palette, diff, choose_files) to
use ShortcutTracker with configurable map entries.
Tests added for Python (5 test methods) and Go (ParseMap + key matching).
The themes kitten used the truncated color name when formatting the
colors themselves, which leads to broken coloring when the window is
narrow enough to cause truncation to occur.
Add new tab named "user" to the built-in themes kitten - new tab
filters and displays only user-defined color themes, identified by
adding "is_user_defined" property to "Theme" class which defaults
to False, and is set True when a theme is fetched using the
"load_from_dir" method.
