A scrollbar you can put anywhere, in every language

[ralyodio]

Closes #62.

The renderer already existed. You could only get one by being a table, list, tree or log, each of which owns its own offset — so a wrapped paragraph, a canvas or a draw() of your own could not show a scrollbar at all.

This lifts drawScrollbar out of table.ts into its own module, gives it the four edges and state the caller owns, and adds ui.scrollbar(...). The dense widgets keep scrollbar: true and route through the same code, so there is one implementation and one appearance. Their fixtures are untouched: the 1046 lines added to widgets.json are all new cases, none moved.

Two things are not just a move

The thumb is sized from the viewport, not the track. For a table those are the same number — the bar is exactly as tall as the rows it describes — which is why the old five-argument form never needed to ask. A bar you place yourself has no such guarantee: twenty cells of track can describe an eight-line window, and sizing from the track then reports the wrong fraction. thumb takes the viewport and the legacy entry point passes the track, so every existing caller renders identically. Two fixtures pin the case where they differ.

Clicking the track moves you. offsetForPosition returns the offset that centres the thumb on the click, and the container reports it as an onScroll delta — so the handler already driving the content drives the bar too, with no second callback to wire up. A bar with no handlers registers no hit region, so it cannot eat a click meant for what is underneath it.

A horizontal bar also uses the half-height glyphs rather than the full block: a run of solid blocks across a row reads as a rule, not a thumb.

Reach

Six ports carry all of it, plus a container method on each builder. C has no widget layer, so it has nothing here. The Ruby, PHP and Perl bridges gained a scrollbar node and COBOL a SCROLLBAR verb — that was the price of the site's promise that every language has a runnable example for every widget. The widget gallery now has 29.

Verified

• 275 TypeScript tests under both bun test and node --test, typecheck clean
• 61 widget scenes matching the reference in Rust, Go, Python, Zig and C++ (was 54)
• 11/11 ctest, the bindings ABI test, and ports/bindings/tests/check.py
• all eleven widget galleries run, including Ruby, PHP and Perl against a locally built bridge
• both COBOL bridges render the new verb identically (diff clean)
• next build for the site

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