I have a standalone PyQGis application that can load different types of GIS data including GeoJson. It uses QgsMapCanvas as map display, I have implemented the canvasMoveEvent to capture and display mouse' coordinate as it moves. This feature stops working after I load in a gis data file without projection and then calling zoomToFullExtent(). I stepped through the code and it seems it simply stopped sending current mouse position, hence the mouse's coordinate will not change in the interface.
If the gis data I load in has a good projection, then it will work fine.