Answered: How to fix mobile friendly errors in SnowFlat theme?

In my environment, the problem was in the Markdown Editor plugin. Once editing or adding an answer with <pre> block, the edit area and the preview area didn't wrap lines and kept the screen very wide (even on desktop displays): 

P.S.: This behavior was detected only by Google mobile usability test (that "clicks" on the "Answer" button). It was not detected when testing the page by common tools like Lighthouse.

I fixed it with (quite dirty) modifying of CSS file 
qa-pluginmarkdown-editorpagedownsample.css:

.wmd-preview pre,
.qa-a-item-content pre {
 width: 100%; /* ADDED */ 
 max-height: 400px;
 max-width: 685px;
 overflow: auto;
}

@media (max-width: 768px) {
 .wmd-preview pre,
 .qa-a-item-content pre {
     max-width: 550px;
 }
}
@media (max-width: 370px) {
 .wmd-preview pre,
 .qa-a-item-content pre {
     max-width: 250px;
 }