I belated realised that the triangular plastic insert on the exterior of each rear door, adjacent to the rear window, was cracked.
For a long time (
years, it seems, according to old photographs), I had mistakenly thought that the hairline crack was the edge of some black "sealing tape". Only a long while later I discovered that there are similar cracks on the left and the right.
Here's an example of a door with such a plastic insert:

And here's an example of a door-frame only, without any plastic insert (or glass, or door handle):

So, some questions.
* What is this component called? (It's hard to search for information, given I don't know what best to call it.)
* Is this a common problem? (Since discovering the issue a couple of months ago, I have looked at other i30's and other cars in general, and haven't noticed any others with the problem.)
* How would it most likely occur? (Being directly thumped? Bumps on road? Window usage? Rattling when driving at speed with window partly/fully down?)
* Are there any concerns besides æsthetics? (Water ingress? Probably not a security issue, based on the second linked photograph, which indicates that a metal backing would exist behind the plastic insert.)
* Would a typical (non-dealership) mechanic be able to source a suitable replacement? (Without replacing the entire door!)
Thanks.