Sketch doesn't always refresh when the Arduino IDE is refocused

When using the Arduino IDE on Windows with an external editor (set as such in the preferences) simply clicking back onto the window doesn't refresh the sketch that is open. As far as I am aware, it must be reopened by clicking on its icon in the toolbar to get it to refresh the sketch.

Start by reproducing this on Windows with an external editor enabled in Preferences, comparing refocusing the IDE with reopening the sketch from its toolbar icon. Done means the open sketch refreshes when the Arduino IDE window is refocused, without requiring the toolbar action.

Written by the indexing model from the issue text.