This patch changes Blend thumbnails to be a capture of the **entire active window**. The idea is that this does a better job representing the project as it is at any moment in time. It is very fast since it does not require rendering the scene, and works with non-rendered projects like Video Editing,
More specifically this changes the current user preference boolean of saving previews into an enum. so u.flag USER_SAVE_PREVIEWS is deprecated, replaced with an enum so the user can select "None", "Screenshot", or "Camera View".
The following capture shows the Preference set to "Camera View", which is the current default type. Bottom right shows how that preview looks in the File Browser:
{F10042058}
The following capture shows the preference set to "Screenshot", default once this patch is applied. Bottom right shows how that preview looks in the File Browser:
{F10042061}
Note that the best way to test, once you have this compiling, is just to open any of your existing files, select "File / Save" to update the thumbnail, then immediately select File / Open to see the new thumbnail in File Browser.