text_view: Let fenced code blocks scroll inside a height cap

[jarviisha]

Closes #3321

Description

A long fenced code block in a chat answer rendered with TextView::markdown inside a gpui::list pushes the rest of the answer off screen. Capping it with TextViewStyle::code_block(StyleRefinement::default().max_h(..)) could only clip it, and setting overflow.y = Scroll on the refinement didn't work properly: the block only scrolled when code_block_actions gave it an id, it had no scrollbar, and one wheel tick scrolled both the code and the parent list.

This follows the existing table pattern (overflow_x: scroll on style.table). When the code_block refinement sets overflow.y to Overflow::Scroll, CodeBlock::render wraps the block with:

• its own ScrollHandle, kept per block with use_keyed_state (block_element_id("codeblock-scroll", span, ix)), as tables do;
• a vertical ScrollableMask, which consumes the wheel in the capture phase so an ancestor gpui::list doesn't scroll on the same event. Once the code reaches its top or bottom edge, the mask lets the wheel through to the list, as native nested scrollers do;
• a vertical Scrollbar.

The mask takes the block's id, as the table's mask does. The mask, the scrollbar and the code_block_actions box are siblings of the scrolled block, so the copy button stays pinned to the block's top-right corner. Scrolling doesn't depend on code_block_actions any more. Without overflow.y set to Overflow::Scroll the rendered tree is unchanged.

docs/ARCHITECTURE.md lists the code block next to the table under "Scrolling and Virtualization".

let mut code_block = StyleRefinement::default().max_h(px(320.));
code_block.overflow.y = Some(Overflow::Scroll);
TextView::markdown(id, text).style(TextViewStyle::default().code_block(code_block))

Known limit, shared with the table scroll layout: if the refinement sets a margin or a fixed width, the mask, scrollbar and actions follow the outer frame rather than the block.

Public API

No items are added, changed or removed. One existing item gains a documented behavior:

• gpui_base::text::TextViewStyle::with_code_block(style: StyleRefinement) -> Self and gpui_component::text::TextViewStyle::code_block(style: StyleRefinement) -> Self: with overflow.y set to Overflow::Scroll and a max height on style, long code scrolls inside the block with its own scrollbar instead of growing it.

How to Test

• cargo test -p gpui-base --lib scrolling_code_block_keeps_wheel_from_the_list: a scrollable TextView (backed by gpui::list) with a capped scrolling code block, run with and without code_block_actions. From the painted text lines it asserts that a wheel over the code scrolls the code by exactly the wheel delta while the paragraph below (the list) and the pinned actions stay put, and that a wheel below the code still scrolls the list. The test fails without this change: the list scrolls under the code block.
• cargo test -p gpui-base --lib text:: and cargo test -p gpui-component pass.
• Checked manually in the story gallery on Linux (Hyprland) with a temporary TextView demo: the block stops at the cap, shows a scrollbar, and the Copy button stays in the top-right corner.

This change touches gpui-base, because CodeBlock and TextView live there.

The code and this description were written with AI assistance (Claude) and reviewed by me.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)

🤖 Generated with Claude Code