fix regression in ace_popup

Author: nightwing

lib/ace/virtual_renderer.js

@@ -816,10 +816,15 @@ var VirtualRenderer = function(container, theme) {
             // lines may have been pushed down. If so, the first screen row will not
             // have changed, but the first actual row will. In that case, adjust 
             // scrollTop so that the cursor and onscreen content stays in the same place.
+            // TODO: find a better way to handle this, that works non wrapped case and doesn't compute layerConfig twice
             if (config.firstRow != this.layerConfig.firstRow && config.firstRowScreen == this.layerConfig.firstRowScreen) {
-                this.scrollTop = this.scrollTop + (config.firstRow - this.layerConfig.firstRow) * this.lineHeight;
-                changes = changes | this.CHANGE_SCROLL;
-                changes |= this.$computeLayerConfig();
+                var st = this.scrollTop + (config.firstRow - this.layerConfig.firstRow) * this.lineHeight;
+                if (st > 0) {
+                    // this check is needed as a workaround for the documentToScreenRow returning -1 if document.length == 0
+                    this.scrollTop = st;
+                    changes = changes | this.CHANGE_SCROLL;
+                    changes |= this.$computeLayerConfig();
+                }
             }
             config = this.layerConfig;
             // update scrollbar first to not lose scroll position when gutter calls resize