Steps to replicate the issue (include links if applicable):

• With edit filter rights create a large edit filter (of the length of en.wikipedia edit filter 614 or longer)
• Open the browser console
• Use "Test this filter against recent edits"
• Press the Test button

What happens?:

• You will see errors at various stages. The critical one is the last: this is the point that the test fails to run. The software returns "true" so no error is displayed.

What's wrong:

• The api is being invoked with GET, this is failing due to the length of the (url-encoded) filter being added to the URL.
• Note that the non-breaking errors indicate that we are probably using other GETs which may be completely un-necessary.

Confirmation test

• Perform the same tests with a short filter, no issue arises.

What should have happened instead?:

• The API should be invoked using POST, the call should succeed and the test results should be returned.
• If the API call fails the user should be notified.

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):

• Tested on Chrome, Brave and Edge.