fix: evenly distribute values on throttleTime(fn, {leading: true, trailing: true})

[sod]

fixes #2727
fixes #3712

reproduction (rxjs 6.5.1) https://stackblitz.com/edit/rxjs-tthiju

Look at the new test it('should handle a busy producer emitting a regular repeating sequence'

      const e1 =   hot('abcdeabcdeabcdeabcdea|');
      const expected = 'a----a----a----a----a|';

That should be the expected behavior for {leading: true, trailing: true}. Thats the same behavior of https://lodash.com/docs/4.17.11#throttle and https://underscorejs.org/#throttle as well.

Right now throttleTime does it more like:

      const e1 =   hot('abcdefabcdefabcdefabcdefa|');
      const expected = 'a----fa----fa----fa----fa|';

[sod]

@benlesh it seem like I always come at the worst time ... when you prepare the next major version :D. But was there the possibility to include this in rxjs7?

[MatthiasKunnen]

Is this not a duplicate of #4727?

[sod]

Oh indeed it is. Haven't seen it. But #4727 schedules the timer after emitting the value. IMO it's important that scheduling always happens upfront, so user code (if slow) doesn't mess with the timings.

[sod]

i give up again, I hope @MatthiasKunnen has more stamina

[MatthiasKunnen]

I got you @sod. I wanna see this through.