SOLUTION FOUND
Lots of searching and testing and false turns while working on this, but finally got a solution - of course it seems "obvious" after one finally sorts out what is going on.
Short answer: values returned by GM Overlay "fromLatLngToDivPixel" function represent the "GM DIV" and hence location on the GM map whereas "fromLatLngToContainerDiv" values represent the "map_canvas DIV" and hence the observed map boundary. So the actual placing of the menu must be based upon values returned by "fromLatLngToDivPixel" but testing its visibility within the map_canvas DIV must be based upon values from "fromLatLngToContainerDiv". Sometimes values returned by the different functions are identical but sometimes not - the former seems to occur when the map is static whereas the latter can occur when the map is moved/dragged.
Long answer:
The relevant part of the original Pearman code is, where "left" and "top" are later used in CSS styling of the menu, to set its position:
var mousePosition=this.getProjection().fromLatLngToDivPixel(this.position_);
var left=mousePosition.x;
var top=mousePosition.y;
if(mousePosition.x>mapSize.width-menuSize.width-this.pixelOffset.x){
left=left-menuSize.width-this.pixelOffset.x;
} else {
left+=this.pixelOffset.x;
}
if(mousePosition.y>mapSize.height-menuSize.height-this.pixelOffset.y){
top=top-menuSize.height-this.pixelOffset.y;
} else{
top+=this.pixelOffset.y;
}
I found the following helpful comments regarding "fromLatLngToContainerPixel" in How to call fromLatLngToDivPixel in Google Maps API V3? , which led me to my fix
... This works perfectly for me initially, but if I pan the map the
pixel positions are not updated. ...
... using fromLatLngToContainerPixel instead of fromLatLngToDivPixel
solved my issue with pixel positions not updating after panning the map.
Roger Ertesvag
Looking at values returned by "fromLatLngToDivPixel", they were sometimes not valid for the map_canvas DIV containing the map (i.e. were sometime larger than the size of that DIV), whereas "fromLatLngToContainerPixel" DID produce apparently valid values. But the seemingly obvious "fix" of simply replacing "fromLatLngToDivPixel" with "fromLatLngToContainerPixel" often produced much weirdness, with the menu not appearing.
I now gather that the "mousePosition" values returned by "fromLatLngToDivPixel" are valid for the location within the "GoogleMaps DIV" (still unclear exactly what that is) but are not appropriate for the map_canvas DIV. Thus my "fix" was to use values from "fromLatLngToDivPixel" for the "base" menu location but use values from "fromLatLngToContainerPixel" to test and alter its display within the map_canvas DIV.
Specifically, instead of the original I'm now using the following code and it is working successfully.
var mousePosition=this.getProjection().fromLatLngToDivPixel(this.position_);
var left=mousePosition.x;
var top=mousePosition.y;
// my fix below, adjusting location based on nearness of map area boundary
var containerPosition=this.getProjection().fromLatLngToContainerPixel(this.position_);
if(containerPosition.x>mapSize.width-menuSize.width-this.pixelOffset.x){
left=left-menuSize.width-this.pixelOffset.x;
} else {
left+=this.pixelOffset.x;
}
if(containerPosition.y>mapSize.height-menuSize.height-this.pixelOffset.y){
top=top-menuSize.height-this.pixelOffset.y;
} else {
top+=this.pixelOffset.y;
}
