Add another hack to ensure a stable scrollback position on text appends with an off-viewport selection, which apparently broke when we found a workaround around QText's problems with reliable paragraph numbering to fix marker line tracking and reliable selection recreation across appends.

Thanks to Modestas Vainius for the patch.

It seems this might screw with keeping off-viewport selections across appends, but it's still an improvement over scrolling around chaotically while reading backlog.

Here's to the day when we can say goodbye to QText*/3 forever.