don't automatically dismiss zero width diagnostics

Normal diagnostics can be removed when all the text they cover has been removed.
This is not the case for zero width diagnostics as these are not attached to any
particular text. These diagnostics can not be automatically dismissed and must
be mapped as a point instead of as a range to avoid that.

helix-view/src/document.rs

@@ -1225,6 +1225,10 @@ impl Document {
             changes.update_positions(self.diagnostics.iter_mut().map(|diagnostic| {
                 let assoc = if diagnostic.ends_at_word {
                     Assoc::AfterWord
+                } else if diagnostic.zero_width {
+                    // for zero width diagnostics treat the diagnostic as a point
+                    // rather than attachted to some particular text
+                    Assoc::After
                 } else {
                     Assoc::Before
                 };