Trimless for Google Mail™ by Alec Mev

Expands trimmed parts, removes "Show trimmed content" buttons, sets trimmed text's color to light gray and indents it to the right. Restores clipped messages as well. You can switch it on and off by simply clicking the scissors icon in the address bar. The color and the size of indentation are fully customizable.

Tinker with the source code on GitHub: https://github.com/alecmev/trimless-gmail