If block rendering is activated, the commit messages will be formated like wiki pages.

Inside wiki pages it's possible to use the fossil internal wiki format combined with pure html. If a commit message uses for example tables, i cannot simply stop rendering at the first end paragraph after n characters. I t can be, that this paragraph is inside a table cell. Cutting the commit message may corrupt the complete gui display.

To handle this situation, fossil has to
  *  parse it's own html - this might be possible
  *  parse all currently unhandled html - we have no code for this!

For me, the effort to handle shortened html is to much for this. The only thing, i could accept would be, to offer the opportunity to shorten the simplified text display but leave the block rendering variant unchanged.