Fastmail shows lists of messages using a progressively-loaded list, where each item is of a consistent height (88px for me, but it can be a few other values too, depending on your configuration—I think 51px is the default). This means that the scrollbar is real and accurate, and you can seek to any point in your mailbox easily (provided your platform allows interacting with the scrollbar, which largely means “on desktop platforms”). But this does cause problems for very large mailboxes, because browsers only support finite lengths.

A few years back, while I worked at Fastmail, we had a ticket come in from an IE user that they could suddenly only access the first few messages in their mailbox. Trouble was they’d gone over IE’s limit, and IE just ignored the entire height declaration in that case, and so you ended up with only the initially-rendered list items available.

The limits I found:

• Firefox: ignores declarations that resolve to a value higher than 17,895,697 pixels (which is a bit more than 2²⁴).

• IE: ignores declarations that resolve to a value equal to or higher than 10,737,418.23 pixels (2³⁰ − 1 hundredth pixels).

• WebKit: clamps values somewhere around 2²⁵ (~33,554,432) pixels; clamping means you don’t need to worry about it so much, since that was the best workaround in other browsers anyway.

And so we ended up with the workaround code at https://github.com/fastmail/overture/blob/41cdf36f3e7c8f0dd1... (the Firefox check was of much older vintage, I just added the IE case). (Nowadays, the IE part is gone again because IE is gone, hooray!)

So yeah, it actually only took about 200,000 messages in the list to hit this limit and fall over, or subsequently just make the bottom of the mailbox inaccessible. 200,000 messages in one mailbox is uncommon, but not at all unrealistic, especially in an “All mail” sort of mailbox.

For those in this thread saying “why would you ever want to do that?”: this is why. Real scrolling is extremely useful in small cases, and still absolutely useful on occasion as things grow even extraordinarily large. This is not a degenerate case: this is very sane. And pagination of any form would be horrible. Like pagination basically just makes old emails inaccessible except by precise search in Gmail. Fastmail’s approach is very, very strongly desirable here. Pagination is the clumsy hack, not progressive loading of a list (though I will admit that you need list items to be of consistent heights to do a good job of this style of progressive loading).