It stores the line
When you save the page, Latr takes the line of text at the top of your screen - skipping sticky headers, navigation and floating tables of contents, which are on screen without being what you are reading.
A tab is a page, an intention, and a place in it. Latr stores the line of text you were on and hands it back to the browser when the page comes back - so a long article resumes rather than restarting.
The problem
You were four thousand words into something long, saved it for tomorrow, and tomorrow the page comes back at the headline. So you scroll, hunting for the paragraph you half remember - and the cost of picking a long read back up is high enough, often enough, that eventually you stop picking them up at all.
How it works
When you save the page, Latr takes the line of text at the top of your screen - skipping sticky headers, navigation and floating tables of contents, which are on screen without being what you are reading.
The line comes back as a text fragment in the address, so the page lands where you were as it loads. No script runs in the tab, and nothing has to wait for the page to finish rendering.
Reading the page happens at the moment you press save, which is a gesture the browser already permits. Latr never asks to read or change your data on the sites you visit, and the line is never sent anywhere.
Why this way
Every other way of doing this stores a number: how far down the page you were. That number is wrong at a different window size, wrong once lazy images have loaded and reflowed everything, and wrong again if the article gained a paragraph - which it has, by the time a reminder fires days later. A line of text is the same line whatever the layout has done since.
When it cannot
A page that has changed too much, a page that renders late, or a local file all fall back to opening the page exactly as before. Nothing errors, nothing is lost, and a page saved near the top stores no anchor in the first place.
Questions
It stores the line of text that was at the top of your screen when you saved the page - a short run of words, nothing else. Reopening uses that as a text fragment in the address, so the browser itself scrolls to it during a navigation Latr was making anyway.
Because a pixel offset is a lie by the time you come back. It is wrong at a different window size, wrong after the page reflows around lazy-loaded images, and wrong again if the article gained a paragraph. A line of text is the same line whatever the layout does. Storing an offset would also need a script running in the reopened tab, which would mean asking to read and change your data on every website - a permission Latr does not have and will not ask for.
Then it opens the page exactly as it would have anyway. Every failure here degrades to the old behaviour rather than to an error: no match, a page that renders late, a local file, or a browser without text-fragment support.
Yes. Firefox reads the same kind of text anchor, though only once a page has loaded, so there Latr opens the page first and then hands it the line; you may see the top of the page for a moment before it scrolls.
No. A page you saved within about a hundred pixels of the top gets no anchor at all, which is most saves - reopening the top of a page you were at the top of is just opening the page.
No. It is stored in your browser with the rest of the reminder. A free install of Latr makes no network requests at all, and the line never leaves the device.
Free, no account, and nothing leaves your device. About ten seconds to install, and one keystroke to use.
Also on Brave, Vivaldi, Arc and Opera - straight from the Chrome Web Store listing.