feat(notes): auto-merge non-overlapping edits on conflict

When a save hits a GitHub conflict, try a three-way merge first: if our
edits and the remote ones don't overlap, commit the result silently with
a tailored toast. Only genuinely overlapping edits open the conflict
modal. Adds an optional successMessage override to updateFile.

package.json

@@ -49,6 +49,7 @@
     "markdown-it-shikiji": "^0.10.2",
     "mermaid": "^11.12.1",
     "nanoid": "^5.1.6",
+    "node-diff3": "^3.2.1",
     "notyf": "^3.10.0",
     "pastel-color": "^1.0.3",
     "pinia": "^2.2.6",

pnpm-lock.yaml

@@ -104,6 +104,9 @@ importers:
       nanoid:
         specifier: ^5.1.6
         version: 5.1.6
+      node-diff3:
+        specifier: ^3.2.1
+        version: 3.2.1
       notyf:
         specifier: ^3.10.0
         version: 3.10.0
@@ -5130,6 +5133,10 @@ packages:
   node-addon-api@7.1.1:
     resolution: {integrity: sha512-5m3bsyrjFWE1xf7nz7YXdN4udnVtXK6/Yfgn5qnahL6bCkf2yKt4k3nuTKAtT4r3IG8JNR2ncsIMdZuAzJjHQQ==}
 
+  node-diff3@3.2.1:
+    resolution: {integrity: sha512-eKZcJ8RtMQ3cIaA15EgmtwG927fYnRlhtdA4Q9HAcCpAZPQhhU2XptnTH9GeAkMiX2bAXxlJSAFa9shFbztPgw==}
+    engines: {bun: '>=1.3.10'}
+
   node-fetch@2.6.12:
     resolution: {integrity: sha512-C/fGU2E8ToujUivIO0H+tpQ6HWo4eEmchoPIoXtxCrVghxdKq+QOHqEZW7tuP3KlV3bC8FRMO5nMCC7Zm1VP6g==}
     engines: {node: 4.x || >=6.0.0}
@@ -12079,6 +12086,8 @@ snapshots:
   node-addon-api@7.1.1:
     optional: true
 
+  node-diff3@3.2.1: {}
+
   node-fetch@2.6.12:
     dependencies:
       whatwg-url: 5.0.0

src/components/StackedNote.vue

@@ -23,6 +23,7 @@ import { encodeUTF8ToBase64 } from "@/utils/decodeBase64ToUTF8"
 import { getFileLanguage, isMarkdownPath } from "@/utils/fileLanguage"
 import { filenameToNoteTitle } from "@/utils/noteTitle"
 import { errorMessage } from "@/utils/notif"
+import { threeWayMerge } from "@/utils/threeWayMerge"
 
 const LinkedNotes = defineAsyncComponent(
   () => import("@/components/LinkedNotes.vue")
@@ -160,7 +161,8 @@ const {
   lastCheckedAt,
   latestSha,
   check: checkFreshness,
-  pullLatest
+  pullLatest,
+  resolveMergeSources
 } = useNoteFreshness({
   user: user.value,
   repo: repo.value,
@@ -220,9 +222,7 @@ const performSave = async (overrideSha?: string) => {
   })
 
   if (conflict) {
-    await checkFreshness()
-    conflictOpen.value = true
-    if (mode.value === "read") toggleMode()
+    await handleConflict()
     return
   }
 
@@ -239,6 +239,47 @@ const performSave = async (overrideSha?: string) => {
   advanceStackTo(newSha)
 }
 
+// On a save/freshness conflict, try a 3-way merge first: if our edits and the
+// remote ones don't overlap, commit the merge silently (just a toast) so the
+// user is never interrupted. Only genuinely overlapping edits open the modal.
+const handleConflict = async () => {
+  if (!path.value) return
+
+  const sources = await resolveMergeSources()
+  if (sources) {
+    const { clean, merged } = threeWayMerge(
+      sources.base,
+      rawContent.value,
+      sources.theirs
+    )
+    if (clean) {
+      const { sha: newSha, conflict } = await updateFile({
+        content: merged,
+        path: path.value,
+        sha: sources.remoteSha,
+        successMessage: "✅ Merged remote changes & saved"
+      })
+      if (!conflict && newSha) {
+        rawContent.value = merged
+        await saveCacheNote(encodeUTF8ToBase64(merged), {
+          editedSha: newSha,
+          path: path.value
+        })
+        initialRawContent.value = merged
+        advanceStackTo(newSha)
+        return
+      }
+      // Remote moved again between fetch and commit — fall back to the modal.
+    }
+  }
+
+  // Overlapping edits, or merge sources unavailable — let the user decide.
+  // Ensure latestSha is set so the modal's "overwrite" has a target.
+  if (!latestSha.value) await checkFreshness()
+  conflictOpen.value = true
+  if (mode.value === "read") toggleMode()
+}
+
 watch(mode, async (newMode) => {
   if (newMode === "edit") {
     void checkFreshness()
@@ -258,7 +299,7 @@ watch(mode, async (newMode) => {
 
   await checkFreshness()
   if (freshnessStatus.value === "outdated") {
-    conflictOpen.value = true
+    await handleConflict()
     return
   }
 
@@ -296,7 +337,7 @@ const onBadgeClick = async () => {
 
     const hasUnsavedEdits = rawContent.value !== initialRawContent.value
     if (hasUnsavedEdits) {
-      conflictOpen.value = true
+      await handleConflict()
       return
     }
 

src/hooks/useGitHubContent.hook.ts

@@ -88,18 +88,20 @@ export const useGitHubContent = ({
   const putFile = async ({
     content,
     path,
-    sha
+    sha,
+    successMessage = "✅ Note saved"
   }: {
     content: string
     path: string
     sha?: string
+    successMessage?: string
   }): Promise<{ sha: string | null; conflict: boolean }> =>
     putRaw({
       contentBase64: encodeUTF8ToBase64(content),
       path,
       sha,
       message: `Updating ${path} from Remanso`,
-      successMessage: "✅ Note saved",
+      successMessage,
       conflictMessage: "⚠ Conflict: this note changed on GitHub",
       failureMessage: "❌ Note could not be saved"
     })
@@ -126,6 +128,7 @@ export const useGitHubContent = ({
       content: string
       path: string
       sha: string
+      successMessage?: string
     }) => putFile(props),
     createFile: async (props: { content: string; path: string }) =>
       putFile(props),

src/hooks/useNoteFreshness.hook.spec.ts

@@ -2,18 +2,27 @@ import { createPinia, setActivePinia } from "pinia"
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest"
 import { ref } from "vue"
 
+import { DataType } from "@/data/DataType.enum"
+
 const {
   fetchLatestSha,
   addFile,
   getRawContent,
   saveCacheNote,
-  queryFileContent
+  queryFileContent,
+  dataGet
 } = vi.hoisted(() => ({
   fetchLatestSha: vi.fn(),
   addFile: vi.fn(),
   getRawContent: vi.fn((s: string) => s),
   saveCacheNote: vi.fn(),
-  queryFileContent: vi.fn()
+  queryFileContent: vi.fn(),
+  dataGet: vi.fn()
+}))
+
+vi.mock("@/data/data", () => ({
+  data: { get: dataGet },
+  generateId: (type: string, id: string) => `${type}-${id}`
 }))
 
 vi.mock("@/hooks/useGitHubContent.hook", () => ({
@@ -232,3 +241,87 @@ describe("useNoteFreshness.pullLatest", () => {
     expect(queryFileContent).toHaveBeenCalledWith("alice", "notes", "cached-sha")
   })
 })
+
+describe("useNoteFreshness.resolveMergeSources", () => {
+  beforeEach(() => {
+    fetchLatestSha.mockReset()
+    queryFileContent.mockReset()
+    dataGet.mockReset()
+    getRawContent.mockReset()
+    getRawContent.mockImplementation((s: string) => s)
+    skipSpinnerWait()
+  })
+
+  afterEach(() => {
+    vi.restoreAllMocks()
+  })
+
+  it("returns null when path is empty", async () => {
+    const { resolveMergeSources } = setup({ path: "" })
+    expect(await resolveMergeSources()).toBeNull()
+  })
+
+  it("resolves base (from cache snapshot) + theirs + remoteSha", async () => {
+    fetchLatestSha.mockResolvedValue({ kind: "ok", sha: "remote-sha" })
+    queryFileContent.mockResolvedValue("THEIRS_B64") // theirs blob
+    dataGet.mockResolvedValue({ content: "BASE_B64" }) // base snapshot in cache
+
+    const { resolveMergeSources, latestSha } = setup({
+      sha: "live",
+      edited: "base-sha"
+    })
+    const result = await resolveMergeSources()
+
+    expect(result).toEqual({
+      base: "BASE_B64",
+      theirs: "THEIRS_B64",
+      remoteSha: "remote-sha"
+    })
+    // base must be read from its own immutable snapshot, never the path pointer
+    expect(dataGet).toHaveBeenCalledWith(`${DataType.Note}-base-sha`)
+    expect(queryFileContent).toHaveBeenCalledWith("alice", "notes", "remote-sha")
+    expect(latestSha.value).toBe("remote-sha")
+  })
+
+  it("falls back to fetching the base blob when not cached", async () => {
+    fetchLatestSha.mockResolvedValue({ kind: "ok", sha: "remote-sha" })
+    dataGet.mockResolvedValue(null) // cache miss for base
+    queryFileContent.mockImplementation((_u, _r, s: string) =>
+      Promise.resolve(s === "remote-sha" ? "THEIRS_B64" : "BASE_FROM_BLOB")
+    )
+
+    const { resolveMergeSources } = setup({ sha: "base-sha" })
+    const result = await resolveMergeSources()
+
+    expect(result?.base).toBe("BASE_FROM_BLOB")
+    expect(queryFileContent).toHaveBeenCalledWith("alice", "notes", "base-sha")
+  })
+
+  it("returns null when the remote sha cannot be resolved", async () => {
+    fetchLatestSha.mockResolvedValue({ kind: "offline" })
+
+    const { resolveMergeSources } = setup()
+    expect(await resolveMergeSources()).toBeNull()
+  })
+
+  it("returns null when the remote (theirs) blob fetch fails", async () => {
+    fetchLatestSha.mockResolvedValue({ kind: "ok", sha: "remote-sha" })
+    queryFileContent.mockResolvedValue(null)
+
+    const { resolveMergeSources, latestSha } = setup()
+    expect(await resolveMergeSources()).toBeNull()
+    // latestSha is still surfaced so a modal fallback's overwrite has a target
+    expect(latestSha.value).toBe("remote-sha")
+  })
+
+  it("returns null when the base blob is unavailable (cache miss + blob null)", async () => {
+    fetchLatestSha.mockResolvedValue({ kind: "ok", sha: "remote-sha" })
+    dataGet.mockResolvedValue(null)
+    queryFileContent.mockImplementation((_u, _r, s: string) =>
+      Promise.resolve(s === "remote-sha" ? "THEIRS_B64" : null)
+    )
+
+    const { resolveMergeSources } = setup({ sha: "base-sha" })
+    expect(await resolveMergeSources()).toBeNull()
+  })
+})

src/hooks/useNoteFreshness.hook.ts

@@ -1,8 +1,11 @@
 import { Ref, ref } from "vue"
 
+import { data, generateId } from "@/data/data"
+import { DataType } from "@/data/DataType.enum"
 import { useGitHubContent } from "@/hooks/useGitHubContent.hook"
 import { markdownBuilder } from "@/hooks/useMarkdown.hook"
 import { prepareNoteCache } from "@/modules/note/cache/prepareNoteCache"
+import { Note } from "@/modules/note/models/Note"
 import { queryFileContent } from "@/modules/repo/services/repo"
 import { useUserRepoStore } from "@/modules/repo/store/userRepo.store"
 
@@ -114,11 +117,52 @@ export const useNoteFreshness = ({
     return { raw: getRawContent(fileContent), failureStatus: null }
   }
 
+  // The base blob (common ancestor) must come from its own immutable snapshot,
+  // never from the path pointer — that holds the latest, which is "theirs".
+  const getCachedBlob = async (blobSha: string): Promise<string | null> => {
+    const note = await data.get<DataType.Note, Note>(
+      generateId(DataType.Note, blobSha)
+    )
+    return note?.content ?? null
+  }
+
+  // Resolves the decoded raw text needed for a 3-way merge: `base` is the
+  // version we edited from (the rejected sha), `theirs` is the current remote.
+  // Returns null when any piece can't be resolved (offline, missing base).
+  const resolveMergeSources = async (): Promise<{
+    base: string
+    theirs: string
+    remoteSha: string
+  } | null> => {
+    if (!path.value) return null
+
+    const { sha: remoteSha } = await resolveRemoteSha(path.value)
+    if (!remoteSha) return null
+    // Surface the resolved sha so a modal fallback's Overwrite can use it.
+    latestSha.value = remoteSha
+    const theirsBlob = await queryFileContent(user, repo, remoteSha)
+    if (!theirsBlob) return null
+
+    const baseSha = (await getEditedSha()) ?? sha.value
+    const baseBlob =
+      (await getCachedBlob(baseSha)) ??
+      (await queryFileContent(user, repo, baseSha))
+    if (!baseBlob) return null
+
+    const { getRawContent } = markdownBuilder(sha.value)
+    return {
+      base: getRawContent(baseBlob),
+      theirs: getRawContent(theirsBlob),
+      remoteSha
+    }
+  }
+
   return {
     status,
     lastCheckedAt,
     latestSha,
     check,
-    pullLatest
+    pullLatest,
+    resolveMergeSources
   }
 }

src/utils/threeWayMerge.spec.ts

@@ -0,0 +1,94 @@
+import { describe, expect, it } from "vitest"
+
+import { threeWayMerge } from "./threeWayMerge"
+
+const base = "line1\nline2\nline3\nline4\nline5"
+
+describe("threeWayMerge", () => {
+  it("merges non-overlapping edits cleanly, keeping both changes", () => {
+    const ours = "OURS1\nline2\nline3\nline4\nline5"
+    const theirs = "line1\nline2\nline3\nline4\nTHEIRS5"
+
+    const { clean, merged } = threeWayMerge(base, ours, theirs)
+
+    expect(clean).toBe(true)
+    expect(merged).toBe("OURS1\nline2\nline3\nline4\nTHEIRS5")
+  })
+
+  it("treats identical edits on both sides as clean (false conflict)", () => {
+    const edited = "line1\nCHANGED\nline3\nline4\nline5"
+
+    const { clean, merged } = threeWayMerge(base, edited, edited)
+
+    expect(clean).toBe(true)
+    expect(merged).toBe(edited)
+  })
+
+  it("flags overlapping edits on the same line as not clean", () => {
+    const ours = "line1\nline2\nOURS3\nline4\nline5"
+    const theirs = "line1\nline2\nTHEIRS3\nline4\nline5"
+
+    const { clean } = threeWayMerge(base, ours, theirs)
+
+    expect(clean).toBe(false)
+  })
+
+  it("returns the base unchanged when neither side edited", () => {
+    const { clean, merged } = threeWayMerge(base, base, base)
+
+    expect(clean).toBe(true)
+    expect(merged).toBe(base)
+  })
+
+  it("returns our edits when only we changed", () => {
+    const ours = "OURS1\nline2\nline3\nline4\nline5"
+
+    const { clean, merged } = threeWayMerge(base, ours, base)
+
+    expect(clean).toBe(true)
+    expect(merged).toBe(ours)
+  })
+
+  it("returns their edits when only they changed", () => {
+    const theirs = "line1\nline2\nline3\nline4\nTHEIRS5"
+
+    const { clean, merged } = threeWayMerge(base, base, theirs)
+
+    expect(clean).toBe(true)
+    expect(merged).toBe(theirs)
+  })
+
+  it("preserves a trailing newline", () => {
+    const baseNl = "a\nb\nc\n"
+    const ours = "A\nb\nc\n"
+    const theirs = "a\nb\nC\n"
+
+    const { clean, merged } = threeWayMerge(baseNl, ours, theirs)
+
+    expect(clean).toBe(true)
+    expect(merged).toBe("A\nb\nC\n")
+  })
+
+  it("conservatively flags edits on immediately-adjacent lines as not clean", () => {
+    // No unchanged line separates the two edits, so node-diff3 groups them
+    // into one region and reports a conflict (git would merge this).
+    const ours = "A\nline2\nline3\nline4\nline5"
+    const theirs = "line1\nB\nline3\nline4\nline5"
+
+    const { clean } = threeWayMerge(base, ours, theirs)
+
+    expect(clean).toBe(false)
+  })
+
+  it("merges added lines on different sides without conflict", () => {
+    const ours = "line1\nOURS-NEW\nline2\nline3\nline4\nline5"
+    const theirs = "line1\nline2\nline3\nline4\nline5\nTHEIRS-NEW"
+
+    const { clean, merged } = threeWayMerge(base, ours, theirs)
+
+    expect(clean).toBe(true)
+    expect(merged).toBe(
+      "line1\nOURS-NEW\nline2\nline3\nline4\nline5\nTHEIRS-NEW"
+    )
+  })
+})

src/utils/threeWayMerge.ts

@@ -0,0 +1,40 @@
+import { diff3Merge } from "node-diff3"
+
+export interface ThreeWayMergeResult {
+  clean: boolean
+  merged: string
+}
+
+/**
+ * Line-level 3-way merge. `ours` and `theirs` are both derived from `base`
+ * (the common ancestor). When the two sets of edits are separated by at least
+ * one unchanged line they merge automatically (`clean: true`); when they
+ * overlap, `clean` is false and the caller should fall back to manual
+ * resolution.
+ *
+ * Conservative by design: node-diff3 groups *consecutive* changed lines into a
+ * single region, so edits to immediately-adjacent lines (no unchanged line
+ * between them) are reported as a conflict even when they don't truly overlap.
+ * This errs toward asking the user rather than risking a wrong auto-merge.
+ *
+ * `merged` is only meaningful when `clean` is true — conflicting regions are
+ * dropped from it, so it must not be committed on a non-clean merge.
+ */
+export const threeWayMerge = (
+  base: string,
+  ours: string,
+  theirs: string
+): ThreeWayMergeResult => {
+  // diff3Merge(a, o, b): a = ours, o = ancestor, b = theirs. The default
+  // excludeFalseConflicts folds identical edits made on both sides.
+  const regions = diff3Merge(ours, base, theirs, { stringSeparator: /\r?\n/ })
+
+  const lines: string[] = []
+  let clean = true
+  for (const region of regions) {
+    if (region.ok) lines.push(...region.ok)
+    else clean = false
+  }
+
+  return { clean, merged: lines.join("\n") }
+}