Why is there no rolling shutter when using a mechanical shutter?
When I have a camera, that can take still images at 10fps with a mechanical shutter, why doesn't it mean the sensor can take images at 10fps electronically without producing rolling shutter?


In order to understand why, we have to take a look at a typical 3T(ransistor) pixel:



This 3T pixel can be used with rolling shutters, but not with (electronic) global shutters. The RST signal will reset the voltage across the photodiode to a positive voltage. When light is detected, that voltage will decreases proportional to the detected photogenerated charges. The annoying part for global shuttering is that we cannot shut off the photodiode. We cannot read out all pixels simultaneously for practical reasons (too many wires, read-out circuits, power, etc.), so during read-out light will keep getting collected in other pixels causing their output to change. Adding a mechanical shutter will make it possible to keep light from reaching the photodiode, bypassing the issue.

In order to implement global shuttering in CMOS, you at least need a 4T pixel:



The TG-signal can be used to close or open the path for detected photogenerated charges to be collected on the gate of \$M_{sf}\$. This is an electronic way of controlling the shutter.

The main disadvantage of global shuttering over rolling shutter is that the time window of capturing becomes shorter. This is explained in the post you mentioned. It is also demonstrated in the following diagram (that I quickly drew up in paint).



The reason why global shuttering is a more viable option nowadays, is because of technological advancements (back-side illumination, reduced noise \$&lt;1e^-\$, ...), usually at the expense of cost.