clear body block caches on grow requests

data/lib/mailapi/mailslice.js

@@ -1427,8 +1427,10 @@ FolderStorage.prototype = {
           // console.log('  dirty block:', blockInfo.blockId);
           continue;
         }
-        // do not discard blocks that overlap mail slices
-        if (blockIntersectsAnySlice(blockInfo))
+        // If a non-body block, do not discard blocks that overlap mail
+        // slices. For bodies, want to always purge as front end may
+        // decide to never shrink a messages slice.
+        if (blockType !== 'body' && blockIntersectsAnySlice(blockInfo))
           continue;
         // console.log('discarding', blockType, 'block', blockInfo.blockId);
         delete blockMap[blockInfo.blockId];
@@ -2620,6 +2622,15 @@ FolderStorage.prototype = {
         doneCallback, progressCallback);
     }.bind(this);
 
+    // The front end may not be calling shrink any more, to reduce
+    // complexity for virtual scrolling. So be sure to clear caches
+    // that are not needed, to avoid a large memory growth from
+    // keeping the header bodies as the user does next/previous
+    // navigation.
+    if (this._mutexQueue.length === 0) {
+      this.flushExcessCachedBlocks('grow');
+    }
+
     // --- request messages
     if (dirMagnitude < 0) {
       dir = FUTUREWARDS;