fix: keep a character Markdown does not trim at the end of a line

CHANGELOG.md

@@ -133,6 +133,7 @@ Leafdown uses lightweight [Keep a Changelog](https://keepachangelog.com/en/1.1.0
 - Keep a link or image title in the quotation marks or parentheses it was written with, so a file holding `[Garden](garden.md 'Garden')` no longer comes back rewritten to double quotes. Editing an image no longer rewrites its title either.
 - Leave whitespace that ends a line out of the saved file, so a space typed at the end of a paragraph, heading, list item, quote, or table cell no longer writes a character that the next open discards and a second save then removes. Markdown drops such whitespace on read, so the space was already lost; the file now says so from the first save. Whitespace elsewhere on a line, a hard break, and whitespace inside fenced code are unchanged, and a space written as ` ` at one of those trimmed positions is now dropped on save for the same reason.
 - Leave whitespace that starts a line out of the saved file, so a space or tab typed at the start of a paragraph, heading, list item, quote, or table cell no longer writes a character reference that survives one open and is gone after the one following it. Markdown drops such whitespace on read, so the character was already lost; the file now says so from the first save. Whitespace elsewhere on a line and inside fenced code is unchanged, as is a character reference naming something Markdown does not trim, such as ` `. A space an author wrote as ` ` at the start of a line is now dropped on save, for the reason one written at the end already is.
+- Keep a character Markdown does not trim where it ends a line, so a no-break space, an em space, or an ideographic space closing a paragraph, heading, list item, quote, or table cell is written back as it was read instead of being deleted on the first save. Such a character renders and reloads wherever it is written, so removing it lost text rather than settling the file. A space or tab at the same position is still left out, and one written as ` ` was never affected.
 
 ## [0.1.0-alpha.1] - 2026-07-10
 

corpus/commonmark/text-and-breaks.md

@@ -61,6 +61,24 @@ This line continues the same paragraph.
 
 Two spaces follow the end of this paragraph.  
 
+## Trailing whitespace a parse keeps
+
+A no-break space ends this paragraph. 
+
+An em space ends this paragraph. 
+
+An ideographic space ends this paragraph.　
+
+### A no-break space ends this heading 
+
+- A no-break space ends this list item. 
+
+> A no-break space ends this quote. 
+
+A no-break space follows the space that ends this paragraph.  
+
+A no-break space precedes the space that ends this paragraph.  
+
 ## Hard breaks using a backslash
 
 A backslash follows this line.\

src/features/editor/tests/markdownCompatibility.test.ts

@@ -1346,6 +1346,120 @@ describe("Line-final whitespace", () => {
     expect(firstSave).toBe("```\ncode \n```\n");
     expect(secondSave).toBe(firstSave);
   });
+
+  // A character CommonMark does not trim is content the line carries, so it reaches the file even
+  // where it sits against the line ending. `TRAILING_WHITESPACE_PATTERN` decides both what is left
+  // out and what `state.safe` never sees, which is why these are asserted through the escape it
+  // would otherwise have skipped rather than through the saved character alone.
+  const NO_BREAK_SPACE = "\u00a0";
+  const EM_SPACE = "\u2003";
+  const IDEOGRAPHIC_SPACE = "\u3000";
+
+  const openThenReload = async (initial: string) => {
+    const opened = await mountEditor(initial);
+    const firstSave = opened.getMarkdown();
+    const reloaded = await mountEditor(firstSave);
+
+    return {
+      firstSave,
+      reloadedText: reloaded.view.state.doc.textContent,
+      secondSave: reloaded.getMarkdown(),
+    };
+  };
+
+  it.each([
+    {
+      expected: `plain${NO_BREAK_SPACE}\n`,
+      initial: `plain${NO_BREAK_SPACE}`,
+      name: "a paragraph",
+    },
+    {
+      expected: `# head${NO_BREAK_SPACE}\n`,
+      initial: `# head${NO_BREAK_SPACE}`,
+      name: "a heading",
+    },
+    {
+      expected: `* item${NO_BREAK_SPACE}\n`,
+      initial: `- item${NO_BREAK_SPACE}`,
+      name: "a list item",
+    },
+    {
+      expected: `> quote${NO_BREAK_SPACE}\n`,
+      initial: `> quote${NO_BREAK_SPACE}`,
+      name: "a blockquote",
+    },
+  ])("keeps a no-break space ending $name", async ({ expected, initial }) => {
+    const { firstSave, secondSave } = await openThenReload(initial);
+
+    expect(firstSave).toBe(expected);
+    expect(secondSave).toBe(firstSave);
+  });
+
+  it.each([
+    { character: EM_SPACE, name: "an em space" },
+    { character: IDEOGRAPHIC_SPACE, name: "an ideographic space" },
+  ])("keeps $name ending a paragraph", async ({ character }) => {
+    const { firstSave, secondSave } = await openThenReload(`plain${character}`);
+
+    expect(firstSave).toBe(`plain${character}\n`);
+    expect(secondSave).toBe(firstSave);
+  });
+
+  it("keeps a no-break space ending a table cell", async () => {
+    const { firstSave, secondSave } = await openThenReload(
+      `| A | B |\n| - | - |\n| C${NO_BREAK_SPACE} | D |`,
+    );
+
+    expect(firstSave).toContain(`C${NO_BREAK_SPACE}`);
+    expect(secondSave).toBe(firstSave);
+  });
+
+  it("reloads the paragraph holding the character the file kept", async () => {
+    const { reloadedText } = await openThenReload(`plain${NO_BREAK_SPACE}`);
+
+    expect(reloadedText).toBe(`plain${NO_BREAK_SPACE}`);
+  });
+
+  // The two classes meet here: the space is what a parse trims and goes, the character beside it
+  // is not and stays, whichever order the line puts them in.
+  it.each([
+    {
+      expected: `plain ${NO_BREAK_SPACE}\n`,
+      initial: `plain ${NO_BREAK_SPACE}`,
+      name: "a space the character follows",
+    },
+    {
+      expected: `plain${NO_BREAK_SPACE}\n`,
+      initial: `plain${NO_BREAK_SPACE} `,
+      name: "a space following the character",
+    },
+    {
+      expected: `plain${NO_BREAK_SPACE}\n`,
+      initial: `plain${NO_BREAK_SPACE}\t`,
+      name: "a tab following the character",
+    },
+  ])("keeps a line ending in $name", async ({ expected, initial }) => {
+    const { firstSave, secondSave } = await openThenReload(initial);
+
+    expect(firstSave).toBe(expected);
+    expect(secondSave).toBe(firstSave);
+  });
+
+  // `state.safe` sees the character now that it is no longer split off the value, so the escapes
+  // it decides read the real line rather than one ending where the character starts.
+  it("leaves a backslash the character separates from the line ending bare", async () => {
+    const { firstSave, secondSave } = await openThenReload(`a\\${NO_BREAK_SPACE}`);
+
+    expect(firstSave).toBe(`a\\${NO_BREAK_SPACE}\n`);
+    expect(secondSave).toBe(firstSave);
+  });
+
+  it("keeps a bare autolink bare before the character", async () => {
+    const { firstSave, secondSave } = await openThenReload(`https://example.com${NO_BREAK_SPACE}`);
+
+    expect(firstSave).toBe(`https://example.com${NO_BREAK_SPACE}\n`);
+    expect(secondSave).toBe(firstSave);
+  });
 });
 
 // A parse trims what a line opens with just as it trims what a line closes with, so writing

src/features/editor/utils/markdownText.ts

@@ -43,9 +43,9 @@ interface PhrasingNode {
   children?: readonly PhrasingNode[];
 }
 
-const TRAILING_WHITESPACE_PATTERN = /\s+$/u;
 // CommonMark trims a space or a tab at a line edge and nothing else, so a no-break space stays
 // a character the line carries rather than whitespace the parse drops.
+const TRAILING_WHITESPACE_PATTERN = /[\t ]+$/u;
 const LEADING_WHITESPACE_PATTERN = /^[\t ]+/u;
 const LINE_ENDING_PATTERN = /[\r\n]$/u;
 // `state.safe` escapes ASCII punctuation and nothing else. Decoding with a wider class would read a