The history being this situation….
Internet Explorer has been a corporate browser for two decades now. And many of us remember the dark ages of web development when we needed to have “IE compatible code” and “web compatible code”.
As many companies invested deeply in the browser building portals that worked with specific versions, Microsoft provided a decade ago a compatibility mode, allowing the browser to “behave” like a former version of itself and stay compatible with websites that had not been updated.
You can set, from your website, this compatibility mode to instruct Internet Explorer which version it should run under and since SharePoint 2013, it was set to version 10.
This made a lot of sense originally as SharePoint had a lot of legacy code that needed to be migrated by the product team before it could run properly under IE 11.
However, as years passed, it was more and more painful, degrading performances, compatibility with modern frameworks and bringing strange rendering behaviors.
…And what changed recently
At 2toLead we started noticing a couple of tenants changing from the version 10 compatibility mode to “edge” (IE11, it was called this way at the time in the transition period) with tenant version 18.104.22.16804.
You can check the version of your tenant by using your browser’s development tools, look at any request to SharePoint and look at the MicrosoftSharePointTeamServices response header.
To check which compatibility mode SharePoint is currently sending to your users you can look in the source of the page and check the X-UA-Compatible metadata.
You can mostly expect performances improvements and better compatibility with modern web standards and frameworks. There might be cases where, because you had to workaround issues with older version of IE, some things might start behaving/looking differently.
Lookout for this change coming to your tenants if you have any customization in place!
Alternatively, your admins should be able to put the SharePoint site in a list and force the compatibility from an admin perspective, that might come at the price of some native SharePoint functionalities not working anymore and should only be used temporary to give you the time to fix the situation. https://docs.microsoft.com/en-us/internet-explorer/ie11-deploy-guide/turn-on-enterprise-mode-and-use-a-site-list
Reminder, Internet Explorer 11 is the only supported browser for client OS now. https://support.microsoft.com/en-us/help/17454/lifecycle-faq-internet-explorer