Fix sticky titles dropdown overlap

Fixes #3572

editions/prerelease/tiddlers/system/stickytitles.tid

@@ -0,0 +1,2 @@
+title: $:/themes/tiddlywiki/vanilla/options/stickytitles
+text: yes

themes/tiddlywiki/vanilla/sticky.tid

@@ -12,5 +12,12 @@ title: $:/themes/tiddlywiki/vanilla/sticky
 	background: ``<<colour tiddler-background>>``;
 	z-index: 500;
 }
+
 ``
+<$list filter="[range[100]]">
+`.tc-story-river .tc-tiddler-frame:nth-child(100n+`<$text text=<<currentTiddler>>/>`) {
+z-index: `<$text text={{{ [[200]subtract<currentTiddler>] }}}/>`;
+}
+`
+</$list>
 </$reveal>