This history tracking stuff really seems to work!

src/actions/actions.ts

@@ -908,7 +908,9 @@ class CommandUndo extends BaseCommand {
   public async exec(position: Position, vimState: VimState): Promise<VimState> {
     const newPosition = await HistoryTracker.goBackHistoryStep();
 
-    vimState.cursorPosition = newPosition;
+    if (newPosition !== undefined) {
+      vimState.cursorPosition = newPosition;
+    }
     vimState.alteredHistory = true;
     return vimState;
   }
@@ -922,7 +924,9 @@ class CommandRedo extends BaseCommand {
   public async exec(position: Position, vimState: VimState): Promise<VimState> {
     const newPosition = await HistoryTracker.goForwardHistoryStep();
 
-    vimState.cursorPosition = newPosition;
+    if (newPosition !== undefined) {
+      vimState.cursorPosition = newPosition;
+    }
     vimState.alteredHistory = true;
     return vimState;
   }

src/history/historyTracker.ts

@@ -128,9 +128,12 @@ class HistoryTrackerClass {
         this.currentHistoryStep.isFinished = true;
     }
 
+    /**
+     * Returns undefined on failure.
+     */
     async goBackHistoryStep(): Promise<Position> {
         if (this.currentHistoryStepIndex === -1) {
-            return;
+            return undefined;
         }
 
         let position: Position;
@@ -145,16 +148,19 @@ class HistoryTrackerClass {
         return position;
     }
 
+    /**
+     * Returns undefined on failure.
+     */
     async goForwardHistoryStep(): Promise<Position> {
         if (this.currentHistoryStepIndex === this.historySteps.length - 1) {
             return undefined;
         }
 
+        this.currentHistoryStepIndex++;
+
         let position: Position;
         let step = this.currentHistoryStep;
 
-        this.currentHistoryStepIndex++;
-
         for (const change of step.changes) {
             position = await change.do();
         }
@@ -168,7 +174,8 @@ class HistoryTrackerClass {
         for (let i = 0; i < this.historySteps.length; i++) {
             const step = this.historySteps[i];
 
-            result += step.changes.map(x => x.text).join(",");
+            result += step.changes.map(x => x.text).join("");
+            if (this.currentHistoryStepIndex === i) { result += "+"; }
             if (step.isFinished) { result += "✓"; }
             result += "| ";
         }