STEPS TO REPRODUCE 1. In Settings, enable "Automatic spell checking by default". 2. Create a new file. Type an incorrect text like "adfsllsdkf" and check that the incorrect text is underlined. 3. Go to "Tools > Highlighting > Other" and enable Diff. 4. Check that the incorrect text like "adfsllsdkf" is still underlined. 5. Save the file as "1.txt". Close the file in Kate. 6. Open the file again. Check that no word is underlined, like if the spell checking didn't work...
I don't have exactly the same behavior, the spell checking is deactivated at step 5, when you save. It reactivates at step 6. If I remember Kate's behavior (but it may not be the same on the tested version), a save resets the document state and the syntax Diff deactivates the spell checking. For me, the problem is that Kate doesn't synchronize the spell checking when you change the syntax. When the document is opened, Kate may have retained the syntax associated with file 1.txt and applied the configuration in accordance with the syntax file: no spell checking. If the problem is specific to the lack of spell checking with the Diff syntax, the commit associated with this change is as follows: avoid spell checking in diffs in most diffs, the spell checking fails due to embedded code just avoid to spell check diffs completely ATM There is no way to force verification other than to create a new syntax file that does not force deactivation.