As anyone who knows how much easier double-click + drag makes text selection (you don't have to aim at word boundaries), I use that feature a lot.
But I'm annoyed that it doesn't work properly in WhatsApp Web. If the last line of a message (almost vertically aligned with message time) doesn't have end punctuation, I can't select the last word. If I insist by dragging all the way to the right, then the last word is selected, but message time is selected too ("18:08" is how time is formatted here, just in case). It's impossible to include the last word without selecting the hour (18) too.
So I stripped down the code until I got the bare minimum to reproduce the issue. See the attachment.
You can't select the last word of the line by double-clicking it. And if you double-click a word before and drag to the end of the line, still the last word wouldn't be selected.
Even if you double-click far to the right of the end of the line and drag to the left, last word is skipped in selection.
Then I ran mozregression and found out that it's a regression. As Mozilla deletes autoland builds from over a year ago, I can't point to the exact change that cause the regression, but this is the range:
Works fine in Blink.
You shoud also notice that if you include an end punctuation in the line, then you can select until the end including the punctuation.