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cca51729a1
This adds a GHOSTTY_SCROLL_VIEWPORT_ROW tag with a `size_t row` member
in the value union. The row is an absolute offset from the top of the
scrollable area, clamped to the active area, in the same row space as
the scrollbar offset so thumb positions round-trip cleanly:
ghostty_terminal_scroll_viewport(term,
(GhosttyTerminalScrollViewport){
.tag = GHOSTTY_SCROLL_VIEWPORT_ROW,
.value = {.row = 42},
});
The tag is appended to the existing enum and the union fits within the
reserved padding, so this is ABI compatible.
This also corrects the docs on GHOSTTY_TERMINAL_DATA_SCROLLBAR: the
getter is amortized O(1) (total is maintained incrementally, the offset
is cached), not "expensive". Since there is intentionally no change
callback, the docs now bless polling per frame or per write batch and
diffing, which is what Ghostty's own renderer does.
Motivation: Embedders building native scrollbars can already read scroll
state via GHOSTTY_TERMINAL_DATA_SCROLLBAR, but the write side only
exposed top/bottom/delta scrolling. Mapping a scrollbar thumb drag to an
absolute position required reading the current offset and computing a
delta, which is two calls that must be sequenced atomically by the
caller.
The core already supports absolute positioning and the macOS app uses it
for scroller drags via the scroll_to_row keybinding; this exposes the
same operation through the libghostty C API.