I'm using a pyqgis script to save multiple images of a map and then stitch the images together to create a larger image. One of the layers is a point layer containing all the labels. I noticed that if a point is outside the current map canvas extent, the label for that point is not shown even if the actual label text falls within the extent of the map canvas. This results in labels being cut off at the edges of the saved images. Is there a way to force labels to be shown even if the feature falls outside the map canvas extent?
In the main project window, use the new map canvas magnifier, introduced in QGIS 2.16. It graphically zooms in on the map without changing the scale, so it should keep the label placement consistent and even show partial labels for features that are off screen.
Screenshot from QGIS 2.16 changelog on qgis.com
In print composer, make a very large map that shows the entire extent of the map. Make the page smaller than the map. Only the part of the map on the page will be exported. Move the map and repeat as needed to cover the entire map.
In this close-up, you can see labels for points that are outside the page are displayed on the map (and will be displayed in the exported image). It might help to add a grid to the map and use the grid lines as guides for where to place the pages.