No need to use actual time source, can just track "logical time"
by bumping up a counter on each thumbnail_cache_maintain_capacity

@@ -1578,1 +1579,4 @@
 
+  /* Discard thumbnail requests that are far enough from viewing area. */
+  rctf rect = timeline_ctx->v2d->cur;
+  rect.xmin -= 30;

@@ -0,0 +36,4 @@
+
+// #define DEBUG_PRINT_THUMB_JOB_TIMES
+
+static ThreadMutex thumb_cache_lock = BLI_MUTEX_INITIALIZER;

@@ -0,0 +50,4 @@
+struct ThumbnailCache {
+  struct FrameEntry {
+    int frame_index = 0;
+    int stream_index = 0;

@@ -59,2 +38,2 @@
-  SEQ_sequence_free(item->scene, item->seq_dupli);
-  MEM_freeN(val);
+  int first_drawable_frame = max_iii(
+      SEQ_time_left_handle_frame_get(scene, seq), seq->start, view_area->xmin);

@@ -94,3 +57,1 @@
-  }
-  if (seq->machine > view_area->ymax) {
-    return false;
+  /* If handle position was displayed, align next frame with `seq->start`. */

@@ -1579,0 +1583,4 @@
+  rect.xmax += 30;
+  rect.ymin -= 2;
+  rect.ymax += 2;
+  seq::thumbnail_cache_discard_requests_outside(timeline_ctx->scene, rect);

@@ -0,0 +42,4 @@
+ * If total amount of resident thumbnails is too large, try to remove oldest-used ones to
+ * keep the cache size in check.
+ */
+void thumbnail_cache_maintain_capacity(Scene *scene);

@@ -0,0 +164,4 @@
+  if (!ELEM(seq->type, SEQ_TYPE_MOVIE, SEQ_TYPE_IMAGE)) {
+    return false;
+  }
+  if ((seq->flag & SEQ_FLAG_SKIP_THUMBNAILS) != 0) {

@@ -0,0 +254,4 @@
+
+void ThumbGenerationJob::ensure_job(const bContext *C, ThumbnailCache *cache)
+{
+  wmWindowManager *wm = CTX_wm_manager(C);

@@ -0,0 +377,4 @@
+
+        /* Add result into the cache (under cache mutex lock). */
+        BLI_mutex_lock(&thumb_cache_lock);
+        ThumbnailCache::FileEntry *val = job->cache_->map_.lookup_ptr(request.file_path);

@@ -0,0 +435,4 @@
+    val = cache.map_.lookup_ptr(key);
+  }
+
+  if (val == nullptr) {

@@ -0,0 +445,4 @@
+  for (int64_t index = 0; index < val->frames.size(); index++) {
+    /* Make video stream mismatch count way more than a frame mismatch. */
+    int score = math::abs(frame_index - val->frames[index].frame_index) +
+                math::abs(seq->streamindex - val->frames[index].stream_index) * 1024;

@@ -0,0 +470,4 @@
+                                    img_width,
+                                    img_height);
+    cache.requests_.add(request);
+    ThumbGenerationJob::ensure_job(C, &cache);

@@ -0,0 +555,4 @@
+    /* Do not remove thumbnails for files used within last 10 updates. */
+    int64_t oldest_time = cache->logical_time_ - 10;
+    int64_t oldest_entries = 0;
+    for (const auto &kvp : cache->map_.items()) {

@@ -0,0 +571,4 @@
+    }
+
+    /* If we're still beyond capacity, remove individual long-unused (but not within
+     * last 100 updates) individual frames. */

@@ -0,0 +586,4 @@
+  BLI_mutex_unlock(&thumb_cache_lock);
+}
+
+void thumbnail_cache_discard_requests_outside(Scene *scene, const rctf &rect)

@@ -844,7 +849,6 @@ enum {
 
   SEQ_CACHE_PREFETCH_ENABLE = (1 << 10),
   SEQ_CACHE_DISK_CACHE_ENABLE = (1 << 11),
-  SEQ_CACHE_STORE_THUMBNAIL = (1 << 12),

@@ -620,3 +625,3 @@
   SEQ_FILTERY = (1 << 4),
   SEQ_MUTE = (1 << 5),
-  SEQ_FLAG_SKIP_THUMBNAILS = (1 << 6),
+  /* SEQ_FLAG_SKIP_THUMBNAILS = (1 << 6), */ /* no longer used */