Release v0.6.20: edit history images and share smooth wheel handling

src/history_preview.h

@@ -0,0 +1,17 @@
+#pragma once
+#include <map>
+#include <set>
+#include <string>
+
+namespace ks {
+// Keep the visible working set even in unusually tall windows. Never evict a
+// visible preview to admit another visible preview: that creates a paint loop.
+template<class T>
+void trimHistoryPreviews(std::map<std::wstring,T> &cache,
+                         const std::set<std::wstring> &visible, size_t limit=32)
+{
+    for(auto it=cache.begin(); cache.size()>limit && it!=cache.end();)
+        if(!visible.contains(it->first)) it=cache.erase(it);
+        else ++it;
+}
+}