ui: fix 2- and 3-clicking after/below an end-of-file selection

2- and 3-clicking after a selection (but not within other text—so either beyond the end of the line or file) should click the selected text.

For example, if the sheet can show 10 lines and there are only two lines of text, and the last line is selected, 2- or 3-clicking anywhere below the last line (but not within the text of the last line) should count as a 2- or 3-click in the selected text. Just try it with Acme to see what I mean.

No source file or test is named. First reproduce the behavior in T using the Acme comparison described in the issue, with a short file whose final line is selected; done means 2- or 3-clicking beyond the end of that line or file acts as clicking the selected text, but clicking within other text does not.

Written by the indexing model from the issue text.