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npub1cc3ha7z055mu0rwwu7806t2wt8mj3pvu0uv5mfp2c50dahaqhczshdalg6andtlongwell-block a1e2a9b113 fix(terminal): pin the DOM->engine sign at the boundary it lives on

Wren caught that the one negation was executed by everything and
asserted by nothing. The frontend test proved a wheel delta reaches IPC
as `lines: -2`; the engine tests proved `scroll(+2)` goes back into
history. Neither ran the conversion between them, so deleting `-lines`
from `terminal_scroll` left both suites green and reversed the terminal
on a real trackpad. Reproduced before fixing: that mutant SURVIVED the
full `buzz-terminal` and `buzz-desktop --lib` suites.

The negation becomes `engine_scroll_lines`, a named function rather than
a `-` in an argument list, because a conversion with no name has nowhere
to hang an assertion. Three tests: the arithmetic with the direction in
the test name, an end-to-end pass that drives a real `SharedTerminal`
through the helper and asserts on the text that lands on screen -- the
unit alone would still pass if the engine read the sign the other way --
and the `i32::MIN` case.

`saturating_neg` rather than unary negation, also Wren's: `lines`
arrives over IPC, and `-i32::MIN` panics in debug and wraps back to
`i32::MIN` in release. The release case is the dangerous one, silently
scrolling the wrong way on a crafted command. Saturating keeps the
direction the caller asked for and lets the engine clamp.

Co-authored-by: tlongwell-block <109685178+tlongwell-block@users.noreply.github.com>
Signed-off-by: tlongwell-block <109685178+tlongwell-block@users.noreply.github.com>

2026-08-02 17:52:24 -04:00