Firefox’s limit is 17,895,697 pixels. Others have a limit less than twice as high, so given you’re aiming for a value way higher than that, it’s not a browser-specific issue, except insofar as Firefox ignores rather than clamping, so you have to detect Firefox and clamp it manually.
In Fastmail’s case (see my top-level comment), making the end of a ridiculously large mailbox inaccessible was considered acceptable. In a spreadsheet, that’s probably not so, so you need to do something different. But frankly I think you needed to use a custom scrollbar anyway, as a linear scrollbar will be useless for almost all documents for anything except returning to the top.
Rendering the content, however, to a canvas is not particularly necessary: make a 4 million pixel square area, hide its scrollbars, render the outermost million pixels of all edges at their edge, and where you’re anywhere in the middle (e.g. 1.7 billion rows in), render starting at 2 million pixels, and if the user scrolls a million pixels in any direction, recentre (potentially disrupting scrolling inertia, but that’s about it). That’s basically perfect, allowing native rendering and scrolling interaction, meaning better behaviour and lower latency.
In Fastmail’s case (see my top-level comment), making the end of a ridiculously large mailbox inaccessible was considered acceptable. In a spreadsheet, that’s probably not so, so you need to do something different. But frankly I think you needed to use a custom scrollbar anyway, as a linear scrollbar will be useless for almost all documents for anything except returning to the top.
Rendering the content, however, to a canvas is not particularly necessary: make a 4 million pixel square area, hide its scrollbars, render the outermost million pixels of all edges at their edge, and where you’re anywhere in the middle (e.g. 1.7 billion rows in), render starting at 2 million pixels, and if the user scrolls a million pixels in any direction, recentre (potentially disrupting scrolling inertia, but that’s about it). That’s basically perfect, allowing native rendering and scrolling interaction, meaning better behaviour and lower latency.