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Consecutive Fenced Code Blocks — auto_shrink Fix (issue #129)

Symptom: In rendered and split view, when a markdown document contains two or more fenced code blocks back-to-back (mixed languages or all the same), only the first block is visible. Subsequent blocks appear blank or are pushed far below the viewport. Cursor movement or edits sometimes seem to "briefly refresh" them, but they revert on the next layout pass.

Visible from v0.2.8 onward because the new viewport-culling pipeline (Task 5) and the per-block height cache (Task 6) latch onto the wrong heights produced by this layout bug and use them as the canonical layout for subsequent frames.

Root cause

EditableCodeBlock, MermaidBlock, and the markdown blockquote / callout renderers each wrap their content in a horizontal-only egui::ScrollArea so that wide content scrolls horizontally instead of breaking max_line_width (see commit 70699c5, "fix: horizontal scrolling for code blocks, mermaid diagrams, and blockquotes"). All four spots used:

egui::ScrollArea::horizontal()
    .id_salt(...)
    .auto_shrink([false, false]) // ← bug
    .show(ui, |ui| { ... });

egui sizes a scroll area's perpendicular axis according to the (scroll_enabled, auto_shrink) truth table in egui::containers::scroll_area::Prepared::end:

inner_size[d] = match (scroll_enabled[d], auto_shrink[d]) {
    (true, true)   => inner_size[d].min(content_size[d]), // shrink to fit
    (true, false)  => inner_size[d],                      // fill available
    (false, true)  => content_size[d],                    // size to content
    (false, false) => inner_size[d].max(content_size[d]), // expand to max
};

For ScrollArea::horizontal() the y-axis has scroll_enabled[1] = false, so auto_shrink[1] = false selects the last branch: max(available_height, content_height). When available_height exceeds the block's natural height (true on every fresh layout pass), the inner scroll area is stretched to fill the rest of the viewport. The first code block consumes almost the entire visible area; the next block starts after that gap and lands below the viewport.

show_rendered_editor in src/markdown/editor.rs then captures y_after - y_before as the measured block height and writes it into the block-height cache and the ViewportCullingState. Subsequent frames take the fast culling path, see the (still-wrong) cached height, and continue to position later blocks far off-screen — hence the "invisible until interaction" behaviour in the report.

Fix

Switch the perpendicular axis to auto_shrink_y = true for every horizontal-only scroll area in the rendered pipeline. The horizontal axis keeps false so wide content still triggers a horizontal scrollbar instead of expanding the layout.

File Wrapper Change
src/markdown/widgets.rs (~3133) code-block content (EditableCodeBlock) auto_shrink([false, false]) → auto_shrink([false, true])
src/markdown/widgets.rs (~4504) mermaid block content (MermaidBlock) same
src/markdown/editor.rs (~4361) blockquote (render_blockquote) same
src/markdown/editor.rs (~4434) callout (render_callout) same

The run-output panel's nested horizontal scroll area (~3406) is intentionally left at [false, false] because it sits inside a vertical ScrollArea with a hard max_height(220.0) cap; the perpendicular sizing is bounded there and the panel is meant to look like a fixed-height pane.

Regression tests (src/markdown/editor.rs::tests)

The visible failure is a layout one and cannot be reproduced in a cargo test-only environment, so the regression tests target the data that the viewport culling and height cache rely on:

Test Verifies
consecutive_fenced_blocks_parse_as_separate_ast_nodes Three back-to-back fenced blocks parse as three top-level CodeBlock nodes with valid 1-indexed line ranges.
block_source_slice_extracts_each_consecutive_block_independently block_source_slice returns each block's own source slice without bleeding into the next block's text.
estimate_block_height_is_finite_and_positive_for_each_block estimate_block_height produces a finite, positive value scaled by the block's line count.
block_height_cache_keys_distinguish_consecutive_blocks The blake3-keyed BlockHeightCache returns each block's own height even when several distinct blocks are inserted in sequence.

If any of these regress, the rendered view's layout pipeline cannot trust the inputs it relies on and the bug can re-emerge through a different code path.

Manual verification

test_md/test_consecutive_code_blocks.md ships a 5-block reproducer (plain, C#, Python, Rust, plain). Open in rendered or split view, scroll, and confirm all five blocks remain visible across viewport changes. Do the same with test_md/test_horizontal_scroll.md (existing) to confirm wide-content horizontal scrolling still works after the auto_shrink change.

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