Ok I did some digging and found that the second issue is because of decimal points here:

In my case the window.end was 100.71469 and the itemBound was 100.71428 and therefore for 0.00041 pixels the next screen was considered to be visible. Rounding to 1 (or even 0) decimal value should fix the issue, I'll open a PR.

About the first bullet, I still don't know I'll keep on digging.

Are you using non deterministic rendering? Rounding off will be a double edged sword because it might fix your use case and break others. Can you share your layout provider implementation?

@naqvitalha The issue is clearly visible on this snack https://snack.expo.io/S1OeHVAQM (if you run it on an Android device).

• First of all the first month visible, is February, when it should in fact be January.
• Second, if you try to go from February to March, the title will be stuck on February.

Also you can check the layout provider on that link too, but here is the actual code:

this.layoutProvider = new LayoutProvider(
      (index) => {
        const date = this.indexToDate(index);
        if (date) {
          const miniCalMonthMoment = momentFromDateWithoutTimezone(date);
          if (miniCalMonthMoment) {
            const monthKey = miniCalMonthMoment.format(TimeFormats.MINICAL_KEY_FORMAT);
            if (props.miniCalData) {
              const dayData = props.miniCalData.getIn([monthKey, 'data']);
              if (dayData) {
                const weekCnt = Math.ceil(dayData.size / 7);
                if (weekCnt >= 6) {
                  this.minicalHeight = props.h * (props.showMonthTitles ? 0.42 : 0.37);
                  return VIEW_TYPES.LARGE_SIZE;
                }
              }
            }
          }
        }
        return VIEW_TYPES.NORMAL_SIZE;
      },

      // Providing heights and widths for given types, doing deterministic way for perf
      (type, dim) => {
        switch (type) {
          case VIEW_TYPES.LARGE_SIZE: {
            // eslint-disable-next-line
            dim.height = props.h * (props.showMonthTitles ? 0.4 : 0.35);
            break;
          }
          case VIEW_TYPES.NORMAL_SIZE:
          default: {
            // eslint-disable-next-line
            dim.height = props.h * (props.showMonthTitles ? 0.48 : 0.42);
            break;
          }
        }
        // eslint-disable-next-line
        dim.width = this.minicalWidth;
      },
    );

I tried the expo. First visible month was January. And I don't even get the second issue. What is happening? Device specific issue?

I'm running this on a Nexus 5X @ 8.1.0, and I don't think it's device specific issue (we had lot's of users reporting the same behaviour on various devices) as we're using that code on production.

• First issue is that the first month is January (because there are 2 onVisibleIndexesChanged being dispatched upon mount (the first one is the right one and the second one is wrong).

• The second issue is that on February, if you try to scroll from February to March, the title will still be February even though March is been shown on the minical. Under the hood what happens is that both the old and new visible indices are the same, thus:

both now and notNow are empty.

Truncating the number after the decimal point fixes both issues for us. What side issues do you think that might create btw?

Check slack once. I've posted a video of my experience

Truncating is a problem in non deterministic rendering where item widths and heights can be very irregular. It would be very hard to figure out if that rounding off will lead to right or wrong. E.g in our search page a lot of time our rendered item heights are like 120.56003... Assuming anything is very hard there

I'm not in front of slack atm, I'll join you in ~40 mins.

Any update?

Not really, I still get that issue. It's a real pain, and I'm using hacks to determine what's visible, and that doesn't always return the right index.
Not sure what I can do. Is there anything I can do on my end to help get to the bottom of this?

So the repo I've created for #145 is the same project I've been having trouble with onVisibleIndexesChanged.

Closing due to inactivity.