Rewind the download window and bump validation on regression.

include/bitcoin/node/impl/chasers/chaser_organize.ipp

@@ -315,13 +315,15 @@ void CLASS::do_organize(typename Block::cptr block, bool prioritized,
     // Checking currency before notify also avoids excessive work backlog.
     if (is_block() || current)
     {
-        if (!bumped_)
+        if (!bumped_ || regress)
         {
             // If at start the fork point is top of both chains, and next candidate
             // is already downloaded, then new header will arrive and download will
             // be skipped, resulting in stall until restart at which time the start
             // event will advance through all downloaded candidates and progress on
-            // arrivals. This bumps validation once for current strong headers.
+            // arrivals. This bumps validation once for current strong headers, and
+            // again on regression, as the candidate above the branch point may
+            // already be downloaded when reorganizing back to a stored branch.
             notify(error::success, chase::bump, add1(branch_point));
             bumped_ = true;
         }

src/chasers/chaser_check.cpp

@@ -344,12 +344,15 @@ void chaser_check::do_regressed(height_t branch_point) NOEXCEPT
 {
     BC_ASSERT(stranded());
 
-    // Inconsequential regression, work isn't there yet.
-    if (branch_point >= position())
+    // Inconsequential regression, neither position nor window is there yet.
+    if (branch_point >= std::max(position(), requested_))
         return;
 
-    // Update position, purge outstanding work, and wait on track completion.
-    set_position(branch_point);
+    // Update position and window, purge outstanding work, and wait on track
+    // completion. The window must follow the position or the gap it awaits
+    // is never requested.
+    set_position(std::min(branch_point, position()));
+    requested_ = advanced_ = position();
     stop_tracking();
     maps_.clear();
     notify(error::success, chase::purge, branch_point);