fix: support multi-line commit messages during interactive rebase

src/git/__tests__/git-service.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect, vi, beforeEach } from 'vitest';
-import { GitService, GitError, binCommitTime } from '../git-service';
+import { GitService, GitError, binCommitTime, buildAmendCommandStr } from '../git-service';
 
 // Access private exec method via prototype for mocking
 function mockExec(service: GitService, fn: (args: string[]) => Promise<string>) {
@@ -1300,3 +1300,35 @@ describe('GitService', () => {
     });
   });
 });
+
+describe('buildAmendCommandStr', () => {
+  const escape = (s: string) => `'${s.replace(/'/g, "'\\''")}'`;
+
+  it('returns --no-edit -m for single-line messages', () => {
+    const cmd = buildAmendCommandStr('hello world', escape);
+    expect(cmd).toBe("git commit --amend --no-edit -m 'hello world'");
+  });
+
+  it('returns printf pipeline for multi-line messages', () => {
+    const cmd = buildAmendCommandStr('subject\n\nbody line', escape);
+    expect(cmd).toContain('printf');
+    expect(cmd).toContain('git commit --amend -F -');
+    expect(cmd).toContain("'subject'");
+    expect(cmd).toContain("''");
+    expect(cmd).toContain("'body line'");
+  });
+
+  it('escapes single quotes in multi-line messages', () => {
+    const cmd = buildAmendCommandStr("it's\nbroken", escape);
+    expect(cmd).toContain("'it'\\''s'");
+    expect(cmd).toContain("'broken'");
+  });
+
+  it('handles message with only newlines (empty lines)', () => {
+    const cmd = buildAmendCommandStr('\n\n', escape);
+    expect(cmd).toContain('printf');
+    const parts = cmd.split(' ');
+    // Three empty strings after split
+    expect(parts.filter(p => p === "''").length).toBe(3);
+  });
+});

src/git/__tests__/integration/merge-rebase.integration.test.ts

@@ -526,6 +526,19 @@ describe('GitService integration — merge / rebase / cherry-pick / revert', ()
       expect(subject).toBe('shiny new subject');
     });
 
+    it('reword preserves multi-line commit message via printf pipeline', async () => {
+      commit(repo.path, 'init');
+      const base = head(repo.path);
+      const c1 = commit(repo.path, 'old subject', { 'a.txt': 'A\n' });
+
+      await svc.interactiveRebase(base, [
+        { action: 'reword', hash: c1, subject: 'old subject', message: 'new subject\n\nbody line one\nbody line two' },
+      ]);
+
+      const fullMsg = runGit(repo.path, ['log', '-1', '--format=%B']).trim();
+      expect(fullMsg).toBe('new subject\n\nbody line one\nbody line two');
+    });
+
     it('squash collapses commits and uses the provided final message', async () => {
       commit(repo.path, 'init');
       const base = head(repo.path);
@@ -546,6 +559,23 @@ describe('GitService integration — merge / rebase / cherry-pick / revert', ()
       expect(subjects[0]).toBe('combined ABC');
     });
 
+    it('squash uses multi-line combined message from printf pipeline', async () => {
+      commit(repo.path, 'init');
+      const base = head(repo.path);
+      const c1 = commit(repo.path, 'A', { 'a.txt': 'A\n' });
+      const c2 = commit(repo.path, 'B', { 'b.txt': 'B\n' });
+      const c3 = commit(repo.path, 'C', { 'c.txt': 'C\n' });
+
+      await svc.interactiveRebase(base, [
+        { action: 'pick', hash: c1, subject: 'A', message: 'combined title\n\n- item one\n- item two' },
+        { action: 'squash', hash: c2, subject: 'B' },
+        { action: 'squash', hash: c3, subject: 'C' },
+      ]);
+
+      const fullMsg = runGit(repo.path, ['log', '-1', '--format=%B']).trim();
+      expect(fullMsg).toBe('combined title\n\n- item one\n- item two');
+    });
+
     it('fixup discards the squashed commits message (no exec amend)', async () => {
       commit(repo.path, 'init');
       const base = head(repo.path);

src/git/git-service.ts

@@ -1223,13 +1223,11 @@ export class GitService {
   private shellEscapeForExec(s: string): string {
     return `'${s.replace(/'/g, "'\\''")}'`;
   }
-
-  private buildMFlags(parts: string[]): string {
-    return parts
-      .map(p => p.trim())
-      .filter(Boolean)
-      .map(p => `-m ${this.shellEscapeForExec(p)}`)
-      .join(' ');
+  /** Build amend command for interactive rebase exec lines.
+   *  Single-line → `git commit --amend --no-edit -m 'msg'`.
+   *  Multi-line → `printf '%s\n' ... | git commit --amend -F -` (POSIX, no temp files). */
+  private buildAmendCommand(message: string): string {
+    return buildAmendCommandStr(message, s => this.shellEscapeForExec(s));
   }
 
   /**
@@ -1260,7 +1258,6 @@ export class GitService {
       );
     }
 
-    // Write todo list to a temp file (avoids shell injection)
     const isSquashLike = (a: string) => a === 'squash' || a === 'fixup';
     const lines: string[] = [];
     let i = 0;
@@ -1293,7 +1290,7 @@ export class GitService {
         // Without this, fixup-only groups would silently discard the user's edited message, and
         // squash groups would inherit git's default-editor combined message.
         if (finalMessage && (messageChanged || userWantsReword)) {
-          lines.push(`exec git commit --amend --no-edit ${this.buildMFlags([finalMessage])}`);
+          lines.push(`exec ${this.buildAmendCommand(finalMessage)}`);
         }
         continue;
       }
@@ -1303,12 +1300,11 @@ export class GitService {
         const msg = (todo.message ?? todo.subject).trim();
         lines.push(`pick ${todo.hash}`);
         if (msg) {
-          lines.push(`exec git commit --amend --no-edit ${this.buildMFlags([msg])}`);
+          lines.push(`exec ${this.buildAmendCommand(msg)}`);
         }
         i++;
         continue;
       }
-
       lines.push(`${todo.action} ${todo.hash}`);
       i++;
     }
@@ -1359,7 +1355,6 @@ export class GitService {
         });
       });
     } finally {
-      // Clean up temp file
       await unlink(todoFile).catch(() => {});
     }
   }
@@ -2091,3 +2086,16 @@ export class GitService {
   }
 
 }
+
+/**
+ * Build a `git commit --amend` command for use in interactive rebase exec lines.
+ * Single-line messages → `--no-edit -m 'msg'`.
+ * Multi-line messages → POSIX printf pipeline to preserve newlines.
+ */
+export function buildAmendCommandStr(message: string, escape: (s: string) => string): string {
+  if (!message.includes('\n')) {
+    return `git commit --amend --no-edit -m ${escape(message)}`;
+  }
+  const parts = message.split('\n').map(l => escape(l));
+  return `printf '%s\\n' ${parts.join(' ')} | git commit --amend -F -`;
+}

webview-ui/src/components/rebase/InteractiveRebase.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-  import { onMount } from 'svelte';
+  import { onMount, tick } from 'svelte';
   import { getVsCodeApi } from '../../lib/vscode-api';
   import { t } from '../../lib/i18n/index.svelte';
   import type { Commit } from '../../lib/types';
@@ -21,6 +21,7 @@
     action: 'pick' | 'squash' | 'fixup' | 'reword' | 'edit' | 'drop';
     hash: string;
     subject: string;
+    body: string;
     newMessage?: string;
   }
 
@@ -56,15 +57,80 @@
     return 'none';
   }));
 
+  function autoresize(node: HTMLTextAreaElement) {
+    function resize() {
+      node.style.height = 'auto';
+      node.style.height = node.scrollHeight + 'px';
+    }
+    // Defer initial resize until after value binding is applied
+    tick().then(resize);
+    node.addEventListener('input', resize);
+    return {
+      destroy() { node.removeEventListener('input', resize); },
+      update() { tick().then(resize); },
+    };
+  }
+
+  function fullMessage(todo: TodoEntry): string {
+    return todo.body ? `${todo.subject}\n\n${todo.body}` : todo.subject;
+  }
+
+  function squashGroupMessage(startIndex: number): string {
+    const parts: string[] = [];
+    let i = startIndex;
+    parts.push(fullMessage(todos[i]));
+    i++;
+    while (i < todos.length && (todos[i].action === 'squash' || todos[i].action === 'fixup')) {
+      if (todos[i].action === 'squash') {
+        parts.push(fullMessage(todos[i]));
+      }
+      i++;
+    }
+    return parts.join('\n\n');
+  }
+
+  /** Fingerprint of a squash group — changes when members or their actions change. */
+  function squashFingerprint(startIndex: number): string {
+    let i = startIndex;
+    let fp = todos[i].hash;
+    i++;
+    while (i < todos.length && (todos[i].action === 'squash' || todos[i].action === 'fixup')) {
+      fp += '|' + todos[i].hash + ':' + todos[i].action;
+      i++;
+    }
+    return fp;
+  }
+
+  const groupPrints = $state<Record<number, string>>({});
+
   $effect.pre(() => {
-    todos.forEach((todo, i) => {
+    let i = 0;
+    while (i < todos.length) {
+      const todo = todos[i];
       const role = squashGroups[i];
-      if (role === 'squash-target' && todo.newMessage === undefined) {
-        todos[i].newMessage = todo.subject;
-      } else if (role !== 'squash-target' && todo.action !== 'reword' && todo.newMessage !== undefined) {
+      if (role === 'squash-target') {
+        const fp = squashFingerprint(i);
+        if (groupPrints[i] !== fp) {
+          // Group composition changed — reset to new combined message
+          groupPrints[i] = fp;
+          todos[i].newMessage = squashGroupMessage(i);
+        } else if (todo.newMessage === undefined) {
+          // First time — initialize
+          todos[i].newMessage = squashGroupMessage(i);
+        }
+        // Clear message on squash members
+        i++;
+        while (i < todos.length && (todos[i].action === 'squash' || todos[i].action === 'fixup')) {
+          todos[i].newMessage = undefined;
+          i++;
+        }
+        continue;
+      }
+      if (todo.action !== 'reword' && todo.newMessage !== undefined) {
         todos[i].newMessage = undefined;
       }
-    });
+      i++;
+    }
   });
 
   onMount(() => {
@@ -75,6 +141,7 @@
           action: 'pick' as const,
           hash: c.hash,
           subject: c.subject,
+          body: c.body,
         }));
         initialOrder = msg.payload.commits.map((c: Commit) => c.hash);
         loading = false;
@@ -97,7 +164,7 @@
   function setAction(index: number, action: TodoEntry['action']) {
     todos[index].action = action;
     if (action === 'reword') {
-      todos[index].newMessage = todos[index].subject;
+      todos[index].newMessage = fullMessage(todos[index]);
     } else {
       todos[index].newMessage = undefined;
     }
@@ -224,13 +291,14 @@
           <div class="todo-content">
             <span class="todo-hash">{todo.hash.substring(0, 7)}</span>
             {#if todo.action === 'reword' || groupRole === 'squash-target'}
-              <input
+              <textarea
                 class="todo-message-input"
-                type="text"
+                rows="1"
                 placeholder={todo.action === 'reword' ? t('rebase.inlineDesc.reword') : t('rebase.inlineDesc.squash')}
                 use:tooltip={todo.subject}
+                use:autoresize
                 bind:value={todos[index].newMessage}
-              />
+              ></textarea>
             {:else}
               <span class="todo-subject truncate" class:dropped-text={todo.action === 'drop'} use:tooltip={todo.subject}>{todo.subject}</span>
               {#if todo.action !== 'pick'}
@@ -536,13 +604,18 @@
   .todo-message-input {
     flex: 1;
     min-width: 0;
-    padding: 1px 6px;
+    min-height: 28px;
+    max-height: 160px;
+    padding: 2px 6px;
     background: var(--vscode-input-background, var(--bg-secondary));
     border: 1px solid var(--vscode-input-border, var(--border-color));
     border-radius: 3px;
     color: var(--vscode-input-foreground, var(--text-primary));
     font-size: inherit;
     font-family: inherit;
+    line-height: 1.3;
+    resize: none;
+    overflow-y: auto;
     outline: none;
   }
 

webview-ui/src/components/rebase/__tests__/InteractiveRebase.test.ts

@@ -141,6 +141,55 @@ describe('InteractiveRebase — action changes', () => {
       expect(inputs.length).toBeGreaterThan(0);
     });
   });
+
+  it('pre-fills the full commit message (subject + body) when selecting reword on a commit with body', async () => {
+    const { container } = render(InteractiveRebase, baseProps);
+    deliverCommits([
+      commit({ hash: 'c1', subject: 'one', body: 'line two\nline three' }),
+      commit({ hash: 'c2', subject: 'two' }),
+    ]);
+    await waitFor(() => container.querySelector('.todo-item'));
+    const badges = container.querySelectorAll<HTMLButtonElement>('.action-badge');
+    await fireEvent.click(badges[0]);
+    const opts = container.querySelectorAll<HTMLButtonElement>('.action-option');
+    await fireEvent.click(Array.from(opts).find(o => o.textContent?.toLowerCase().includes('reword'))!);
+    await waitFor(() => {
+      const textarea = container.querySelector<HTMLTextAreaElement>('.todo-message-input');
+      expect(textarea).not.toBeNull();
+      expect(textarea!.value).toContain('line two');
+      expect(textarea!.value).toContain('line three');
+      expect(textarea!.value).toContain('one');
+    });
+  });
+
+  it('shows combined subject+body for squash-target, excluding fixup messages', async () => {
+    const { container } = render(InteractiveRebase, baseProps);
+    deliverCommits([
+      commit({ hash: 'a', subject: 'target', body: 'target body' }),
+      commit({ hash: 'b', subject: 'squash msg', body: 'squash body' }),
+      commit({ hash: 'c', subject: 'fixup msg', body: 'fixup body' }),
+    ]);
+    await waitFor(() => container.querySelector('.todo-item'));
+    const badges = container.querySelectorAll<HTMLButtonElement>('.action-badge');
+    // Squash commit b, fixup commit c
+    await fireEvent.click(badges[1]);
+    const squashOpts = container.querySelectorAll<HTMLButtonElement>('.action-option');
+    await fireEvent.click(Array.from(squashOpts).find(o => o.textContent?.toLowerCase().includes('squash'))!);
+    await fireEvent.click(badges[2]);
+    const fixupOpts = container.querySelectorAll<HTMLButtonElement>('.action-option');
+    await fireEvent.click(Array.from(fixupOpts).find(o => o.textContent?.toLowerCase().includes('fixup'))!);
+    await waitFor(() => {
+      const textarea = container.querySelector<HTMLTextAreaElement>('.todo-message-input');
+      expect(textarea).not.toBeNull();
+      expect(textarea!.value).toContain('target');
+      expect(textarea!.value).toContain('target body');
+      expect(textarea!.value).toContain('squash msg');
+      expect(textarea!.value).toContain('squash body');
+      // fixup messages should be excluded
+      expect(textarea!.value).not.toContain('fixup msg');
+      expect(textarea!.value).not.toContain('fixup body');
+    });
+  });
 });
 
 describe('InteractiveRebase — submit', () => {