Gitlab is essentially a website, and Microsoft Edge is just Chrome. I think there isn't any reason to test on windows anymore.
The browser is wired into the OS in real ways, and they matter. Everything from "How does the os manage passwords" or "how are system notifications presented" or "How much screen real estate do I have have on a given window size (scrollbars...)" to subtle differences in how the network stack behaves, or how windows are rendered.
Hell, I can list about 20 differences just off the top of my head - and a damn good part of that is because I've been forced to pay attention to each of those platforms. Which clearly you have not...
I read your comment as "I only use a mac, and I don't understand".
Overall i'm pretty sure that "list about 20 differences" is relevant in some cases/procucts/websites, but not for gitlab - they just proved that by disallowing working on Windows (which wouldn't disallow testing on a VM).
But here's the important question: are the problems that are caused by ignoring the aforementioned differences enough to prevent people from being able to use your software? Or, conversely, are they enough to weigh more heavily than the functionality that you provide for your users?
Because somehow even Electron based software is pretty popular in the desktop, oftentimes looking and feeling rather different from most of the native software. But apart from nitpicks and rightfully complaining about those annoying inconsistencies, does it really matter enough to spend resources towards fixing them?
I can't think of that many inconsistencies that caused web applications to be unusable across different platforms altogether, apart from IE not supporting certain JavaScript functionality back in the day and non-transpiled (to say, ES5) codebases breaking altogether. But look and feel related things have hardly been as important.
For example, we ran into a difference between Chrome on Linux vs Chrome on macOS where it turns out that on macOS, if you right-click on a link (to open a context menu) it selects the text of the link, while on Linux it does no such thing. Initially we'd tested on only one platform because we assumed they'd behave the same, and this difference resulted in buggy behavior on the untested platform.
I've seen more than once a mac design use some font in a light weight that comes across almost invisible on Windows. I'm the only windows guy at our shop and "everyone has access to windows VMs to test" (lol)
Outside of that, now that Edge is on Chromium most of our edge cases seem to be Safari these days (everyone uses chrome/firefox except a couple management using Safari).
Which all in all I guess is a testament to our lack of full browser testing :)
¯\_(ツ)_/¯
In fact, if you paste text into GitLab's "Web IDE" in a windows browser, the results are (or at least used to be? We stopped using it because of that) horrible to the max. Of course with the added nuance that copy-pasting a word vs. a paragraph vs. an entire file produced vastly different results, too. That one was fun to figure out.