So if the electrical length between the two sections is very short, the two reflected waves just cancel each other out and you get no total reflections.
This is good to know 
That's not how it works.
The wavefront can not guess the future, so it will reflect from the first impedance change (100 to 200 ohms step), no matter how long or short the 200 ohms trace is. Only a part of the energy will be able to continue forward past the 100 to 200 ohms change, and an even lower part of that energy that managed to travel through the 200 ohm trace will bump into the 200 to 100 ohms boundary and reflect back (the second reflection of our initial wavefront).
From the second reflection, a fraction of the energy will continue to travel to the receiver, and a small part will reflect back, where will meet again the first impedance step, and it will split again, back and forth, and so on.
Notice that the second reflection indeed will have the opposed sign relative to the first reflection, but it will also have a small time delay relative to the first reflection (because of the length of the 200 ohms trace), but what's most important, the second reflection will have a much lower amplitude than the first reflection, so the second reflection won't cancel the first one.