openlayers-2
I have a question on how I can change this code
selectedFeature = feature;
popup = new OpenLayers.Popup.FramedCloud("chicken",
feature.geometry.getBounds().getCenterLonLat(),
null,
div,
null, null, onPopupClose);
feature.popup = popup;
to make the center point of popup window exactly where I've clicked, instead of on the center of object bounds as it is now ?
I have no time to check your code but, indeed, handling multiple events is not that easy with openlayers. I also had to struggle with that.
These two links could hopefully help you or at least give you some ideas to find a workaround : http://dev.openlayers.org/examples/highlight-feature.html
I would personally go with features over markers. Point features (and all features) allow for labeling and can be represented in different shapes and colors like circles, x, square and so forth. Not to mention all the additional intrinsic functionality and flexibility features have over markers such as geographical calculations, transformations, click/highlight/select handlers and on and on.
I hate to be pushy, but it would be beneficial if you used something like GeoServer or MapServer on the back end. You could set a WFS strategy that works on your 5-30 second (or whatever) interval timer and allow it to fetch and render the features automatically. In case you're wondering, it already does this in ajax form. So you don't have to write the code to query the database, parse, convert, etc... on 300 objects on your map. Openlayers will render the latest WFS data on your Geoserver which is pointed to PostGIS. You would save yourself a lot of time and effort reinventing that wheel. Openlayers WFS protocol can be set to query based on your viewport on a temporal filter.
Though features don't perform well when there are a lot of them. 300 might be pushing it. If you see that performance is an issue, you can easily switch to using WMS (only if you have Geoserver or Mapserver). You can also use a cluster strategy to minimize un necessary features on your map.
Best Answer
I have a not tested proposition :
1 - From that example you will be able to get coordinates from click -see also that example.
2 - You will, then be able to convert coordinates to pixel, with that function.
3 - Finally, you move the popup with popup.moveTo(new OpenLayers.Pixel(xxx));.
You probably have to calculate it before, regarding popup width and height. With the help of jQuery it can even be dynamic.