Split update summary on big deletes

services/track-changes/app/coffee/UpdatesManager.coffee

@@ -219,11 +219,33 @@ module.exports = UpdatesManager =
 			return !!user_id.match(/^[a-f0-9]{24}$/)
 
 	TIME_BETWEEN_DISTINCT_UPDATES: fiveMinutes = 5 * 60 * 1000
+	SPLIT_ON_DELETE_SIZE: 16 # characters
 	_summarizeUpdates: (updates, existingSummarizedUpdates = []) ->
 		summarizedUpdates = existingSummarizedUpdates.slice()
+		previousUpdateWasBigDelete = false
 		for update in updates
 			earliestUpdate = summarizedUpdates[summarizedUpdates.length - 1]
-			if earliestUpdate and earliestUpdate.meta.start_ts - update.meta.end_ts < @TIME_BETWEEN_DISTINCT_UPDATES
+			shouldConcat = false
+
+			# If a user inserts some text, then deletes a big chunk including that text,
+			# the update we show might concat the insert and delete, and there will be no sign
+			# of that insert having happened, or be able to restore to it (restoring after a big delete is common).
+			# So, we split the summary on 'big' deletes. However, we've stepping backwards in time with
+			# most recent changes considered first, so if this update is a big delete, we want to start
+			# a new summarized update next timge, hence we monitor the previous update.
+			if previousUpdateWasBigDelete
+				shouldConcat = false
+			else if earliestUpdate and earliestUpdate.meta.start_ts - update.meta.end_ts < @TIME_BETWEEN_DISTINCT_UPDATES
+				shouldConcat = true
+
+			isBigDelete = false
+			for op in update.op or []
+				if op.d? and op.d.length > @SPLIT_ON_DELETE_SIZE
+					isBigDelete = true
+
+			previousUpdateWasBigDelete = isBigDelete
+
+			if shouldConcat
 				# check if the user in this update is already present in the earliest update,
 				# if not, add them to the users list of the earliest update
 				earliestUpdate.meta.user_ids = _.union earliestUpdate.meta.user_ids, [update.meta.user_id]

services/track-changes/test/unit/coffee/UpdatesManager/UpdatesManagerTests.coffee

@@ -761,3 +761,41 @@ describe "UpdatesManager", ->
 					start_ts: @now
 					end_ts:   @now + 30
 			}]
+
+		it "should split updates before a big delete", ->
+			result = @UpdatesManager._summarizeUpdates [{
+				doc_id: "doc-id-1"
+				op: [{ d: "this is a long long long long long delete", p: 34 }]
+				meta:
+					user_id: @user_1.id
+					start_ts: @now + 20
+					end_ts:   @now + 30
+				v: 5
+			}, {
+				doc_id: "doc-id-1"
+				meta:
+					user_id: @user_2.id
+					start_ts: @now
+					end_ts:   @now + 10
+				v: 4
+			}]
+
+			expect(result).to.deep.equal [{
+				docs:
+					"doc-id-1":
+						fromV: 5
+						toV: 5
+				meta:
+					user_ids: [@user_1.id]
+					start_ts: @now + 20
+					end_ts:   @now + 30
+			}, {
+				docs:
+					"doc-id-1":
+						fromV: 4
+						toV: 4
+				meta:
+					user_ids: [@user_2.id]
+					start_ts: @now
+					end_ts:   @now + 10
+			}]