Log viewer's "ShowMap" is not showing map when log is first opened

For some reason if "Show Map" is checked when a log is first opened the map tiles don't seem to be loaded. If I zoom out and back in again the map appears.

Here is a screen shot of the log viewer immediately after opening a log.

Here it is a moment later after i've used the mount scroll wheel to zoom out and back in again (just a single position change of the mouse's scroll wheel is enough)

Reproduce the issue in Mission Planner's log viewer with Show Map checked before opening a log, then compare the initial view with the view after a small zoom change. Trace the log-opening and map-display entry points to determine why tiles are not loaded initially; done means the map tiles appear immediately when the log opens.

Written by the indexing model from the issue text.