test: add regression examples for visual rendering

Author: ImmanuelHaffner

test/regression-examples.md

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+# Regression Testing Examples
+
+Instructions: Open this file with `set wrap` enabled. Walk through each
+numbered section matching the regression matrix.
+
+---
+
+## 1 — Concealed content: table with long URLs renders
+
+The table below has long URLs that get concealed. It should still render
+as a proper table (borders, padding, alignment) — not fall back to raw text.
+
+| Feature | Status | Docs |
+|---------|--------|------|
+| **Bold** | ✅ Done | [spec](https://spec.commonmark.org/0.31.2/#emphasis-and-strong-emphasis-with-asterisks-and-underscores-rule-1-through-17) |
+| `code` | ⚠️ Partial | [GFM](https://github.github.com/gfm/#strikethrough-extension-with-tildes-and-double-tildes-for-del-elements) |
+| Nested lists | 🔧 WIP | [deep](https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#nested-lists-ordered-and-unordered-mixing-indentation-levels) |
+
+---
+
+## 2 — Wrap continuation lines show table borders
+
+Shrink the window width until the table above wraps. Continuation lines
+should show `│...│` table borders — not raw text leaking through.
+
+(Use the same table from §1 above. Narrow the window to ~60 columns.)
+
+---
+
+## 3 — Strikethrough doesn't inflate column widths
+
+The `~~Strikethrough~~` column should be the same visual width as the text
+without the `~~` markers. Compare column widths with and without.
+
+| Style | Example | Notes |
+|-------|---------|-------|
+| Plain | hello | baseline |
+| ~~Strikethrough~~ | ~~deleted~~ | width should match "deleted" |
+| **Bold** | **strong** | width should match "strong" |
+
+---
+
+## 4 — Table borders use MarkviewTable* highlights
+
+All table borders (`│`, `─`, `┼`, `╭`, `╮`, etc.) should use `MarkviewTableBorder`
+or `MarkviewTableHeader` highlight groups — not treesitter `@punctuation.special`.
+
+Inspect the borders of this table:
+
+| A | B |
+|---|---|
+| 1 | 2 |
+
+---
+
+## 5 — Right border on first screen row of wrapping lines
+
+When a table row wraps, the **first** screen row of that wrapped line should
+still have a right border `│` at the correct position.
+
+(Use the long-URL table from §1. Shrink window until rows wrap. Check the
+right edge of the first screen row of each wrapping line.)
+
+---
+
+## 6 — Top/bottom border indent for nested tables (list context)
+
+The table below is inside an ordered list. The top and bottom borders should
+be indented to align with the table content — not flush-left.
+
+1. Here is an item with a nested table:
+
+   | Key | Val |
+   |-----|-----|
+   | `x` | 42  |
+   | `y` | 99  |
+
+2. Another list item.
+
+And a deeper nesting:
+
+1. Level 1
+   - Level 2
+     1. Level 3 table:
+
+        | A | B | C |
+        |---|---|---|
+        | 1 | 2 | 3 |
+
+     2. Back to list.
+
+---
+
+## 7 — Separator decorations stable in hybrid mode
+
+Move your cursor in and out of the table below. The separator row decorations
+(`─`, `╶`, `┼`) should **not** swap order when the cursor enters/leaves.
+
+| Left | Center | Right |
+|:-----|:------:|------:|
+| aaa | bbb | ccc |
+| ddd | eee | fff |
+
+Also test with cursor on this heading, then move into the table above.
+
+---
+
+## 8 — Tables inside blockquotes render fully
+
+The table inside this blockquote should render the separator row AND all
+data rows — not just the header.
+
+> | Name | Value |
+> |------|-------|
+> | alpha | 1 |
+> | beta | 2 |
+> | gamma | 3 |
+
+Nested blockquote:
+
+> > | X | Y |
+> > |---|---|
+> > | a | b |
+
+---
+
+## 9 — Simple table renders correctly (nowrap)
+
+Set `nowrap`. This simple table should render with proper borders.
+
+| One |
+|-----|
+| 1   |
+
+And a wider one:
+
+| Col A | Col B | Col C | Col D |
+|-------|-------|-------|-------|
+| foo | bar | baz | qux |
+| alpha | beta | gamma | delta |
+
+---
+
+## 10 — Alignment markers render correctly
+
+Each column should show the correct alignment decoration in the separator row.
+
+| Default | Left | Center | Right |
+|---------|:-----|:------:|------:|
+| none | left | center | right |
+| aaa | bbb | ccc | ddd |
+
+---
+
+## 11 — Blockquote borders alongside table borders
+
+Both the blockquote border (`▋`) and table borders (`│`) should be visible
+side by side.
+
+> | Animal | Sound |
+> |--------|-------|
+> | Cat | Meow |
+> | Dog | Woof |
+
+---
+
+## 12 — Hybrid mode: cursor-line un-renders/re-renders cleanly
+
+Move your cursor row-by-row through this table. Each row should un-render
+when the cursor is on it (showing raw markdown) and re-render when the
+cursor leaves.
+
+| Language | Typing | Speed |
+|----------|--------|-------|
+| Lua | dynamic | fast |
+| Rust | static | fast |
+| Python | dynamic | moderate |
+| C | static | very fast |
+
+---
+
+## 13 — Code blocks inside lists render correctly
+
+The code block below is nested inside a list. It should render with proper
+syntax highlighting and code-block decorations.
+
+1. **First item**
+   - Sub-item with code:
+
+     ```lua
+     local M = {}
+     function M.setup(opts)
+       return vim.tbl_deep_extend("force", {}, opts or {})
+     end
+     return M
+     ```
+
+   - Another sub-item
+
+2. **Second item**
+
+---
+
+## 14 — Headings, horizontal rules, inline formatting unaffected
+
+### This is an H3
+
+#### This is an H4
+
+##### This is an H5
+
+---
+
+Inline formatting: **bold**, *italic*, ***bold-italic***, `inline code`,
+~~strikethrough~~, and [a link](https://example.com).
+
+A horizontal rule below:
+
+---
+
+And another:
+
+***
+
+---
+
+## End of regression examples
+
+Replace ☐ with ✅ or ❌ in `test/regression-matrix.md` as you test each case.