If you have a custom MasterPage in SharePoint Online and you have not tracked changes back in from the OOB MasterPage that it is derived from, you may want to do so now.
At some point the Seattle MasterPage has been updated to run IE 11 in IE 10 mode. Take a look at the code example below for how the MasterPage used to handle IE 11:
Notice the X-UA-Compatible http-equiv meta tag and how it says IE 11 should load as IE 11.
Now, this can actually cause some pretty serious issues. For example if you have a Publishing page that contains a list view WebPart, loading a document library view, it will crash IE 11 (when the above meta tag is present)!
Microsoft have “resolved” this issue by telling IE 11 to render in IE 10 mode, amending the Seattle MasterPage http-equiv meta tag as follows:
Update your custom MasterPage to ensure that it plays nice with IE 11.