thin lines between objects that have supposedly been snapped together

Quote from sadrobot

I am trying to make a tessellation using a grid of rectangles, snapped together, although the tessellation itself is composed of white and yellow triangles rather than rectangles. Each triangle spans across two rectangles. The problem I am having is that, if there is a dark background, where the two rectangles join, a dark line is visible, even though the two rectangles have been snapped together and therefore there should be nothing in between them. Sometimes the line disappears when I zoom right in, but appears when I zoom out. The behaviour is a bit inconsistent.

I've tried resizing the rectangles to whole numbers of pixels. I've tried increasing the resolution of the design in AF Photo. Nothing seems to work.

How can I get rid of these lines/gaps?

The way you've put it, you're saying everything and nothing at the same time—we don't know how you created these triangles;
do they have a stroke?
are they vectors, or have they been converted to curves?
a screenshot of the situation, including the Layers panel, would be helpful;
are you working on Mac or Windows?
Basically, give us as much information as possible so we don't have to guess (it's too hot for that)...

You would also have to check, and possibly experiment, in the snapping options. The less are selected, the easiest it is to achieve the expected result.

You could also consider the possibility to use a triangular or oblique grid (Mode > Advanced). Note that you can edit the grid presets.

This is a display error caused by the design of the monitor panels.

Monitors can only display integer pixels. As a result, sub-pixel gaps may appear at zoom levels < or > 100%.

It may help to set Retina Rendering to “High Quality (Slowest)” in Affinity's performance settings.