Merge pull request #35731 from overleaf/mg-historyot-delete-at-end

Fix History OT base length when a tracked delete is in the document

GitOrigin-RevId: 414349aedd498c2635bb3067388dcc7291e32ee6

services/web/frontend/js/features/source-editor/extensions/history-ot.ts

@@ -251,8 +251,11 @@ const updateSender = EditorState.transactionExtender.of(tr => {
   const trackingUserId = tr.startState.field(trackChangesUserIdState)
   const trackedDeletes = trackedDeletesFromState(tr.startState)
   const startDoc = tr.startState.doc
+  // Seed the builder with the full snapshot length (visible length plus all
+  // tracked deletes). Mapping the CM length through toSnapshot under-counts
+  // when a tracked delete sits at the end of the document.
   const opBuilder = new OperationBuilder(
-    trackedDeletes.toSnapshot(startDoc.length)
+    startDoc.length + trackedDeletes.totalLength
   )
 
   if (trackingUserId == null) {

services/web/frontend/js/features/source-editor/utils/tracked-deletes.ts

@@ -13,6 +13,12 @@ export class TrackedDeletes {
     toSnapshot: OffsetTable
   }
 
+  /**
+   * Total length of tracked deletes, i.e. the difference between the snapshot
+   * length and the visible CodeMirror length.
+   */
+  readonly totalLength: number
+
   constructor(trackedChanges: TrackedChangeList) {
     this.offsets = {
       toCM6: [{ pos: 0, map: pos => pos }],
@@ -42,12 +48,20 @@ export class TrackedDeletes {
         offset = newOffset
       }
     }
+    this.totalLength = offset
   }
 
   toCodeMirror(snapshotPos: number) {
     return this.mapPos(snapshotPos, this.offsets.toCM6)
   }
 
+  /**
+   * Map a CodeMirror position to a snapshot position, for placing an edit. At a
+   * tracked-delete boundary this deliberately maps to *before* the delete (so
+   * typing lands before the struck-through text), which means it under-counts
+   * for a delete at the end of the document. Use `totalLength` for the
+   * whole-document length, not `toSnapshot(docLength)`.
+   */
   toSnapshot(cm6Pos: number) {
     return this.mapPos(cm6Pos, this.offsets.toSnapshot)
   }