Add option to show the most severe diagnostics per line

autoload/lsp/internal/diagnostics/virtual_text.vim

@@ -159,10 +159,13 @@ function! s:set_virtual_text(params) abort
 endfunction
 
 function! s:place_virtual_text(server, diagnostics_response, bufnr) abort
+    let l:line_props = {}
+
     let l:linecount = s:Buffer.get_line_count(a:bufnr)
     for l:item in lsp#utils#iteratable(a:diagnostics_response['params']['diagnostics'])
         let l:line = lsp#utils#position#lsp_line_to_vim(a:bufnr, l:item['range']['start'])
-        let l:name = get(s:severity_sign_names_mapping, get(l:item, 'severity', 3), 'LspError')
+        let l:severity = get(l:item, 'severity', 3)
+        let l:name = get(s:severity_sign_names_mapping, l:severity, 'LspError')
         let l:text = g:lsp_diagnostics_virtual_text_prefix . l:item['message']
 
         " Some language servers report an unexpected EOF one line past the end
@@ -179,16 +182,30 @@ function! s:place_virtual_text(server, diagnostics_response, bufnr) abort
             " it's an error to add virtual text on lines that don't exist
             " anymore due to async processing, just skip such diagnostics
             if l:line <= l:linecount
+                if g:lsp_diagnostics_virtual_text_tidy && has_key(l:line_props, l:line)
+                  " Replace the existing virtual text with the one that has higher severity
+                  if l:severity <= l:line_props[l:line]['severity']
+                    call prop_remove({'id': l:line_props[l:line]['prop_id']}, l:line)
+                  else
+                    continue
+                  endif
+                endif
+
                 let l:type = 'vim_lsp_' . l:name . '_virtual_text'
                 call prop_remove({'all': v:true, 'type': l:type, 'bufnr': a:bufnr}, l:line)
-                call prop_add(
+                let l:prop_id = prop_add(
                 \ l:line, 0,
                 \ {
                 \   'type': l:type, 'text': l:text, 'bufnr': a:bufnr,
                 \   'text_align': g:lsp_diagnostics_virtual_text_align,
                 \   'text_padding_left': g:lsp_diagnostics_virtual_text_padding_left,
                 \   'text_wrap': g:lsp_diagnostics_virtual_text_wrap,
                 \ })
+
+                let l:line_props[l:line] = {
+                \ 'prop_id': l:prop_id,
+                \ 'severity': l:severity,
+                \ }
             endif
         endif
     endfor

doc/vim-lsp.txt

@@ -57,6 +57,8 @@ CONTENTS                                                  *vim-lsp-contents*
 			     |g:lsp_diagnostics_virtual_text_padding_left|
       g:lsp_diagnostics_virtual_text_wrap
 			     |g:lsp_diagnostics_virtual_text_wrap|
+      g:lsp_diagnostics_virtual_text_tidy
+			     |g:lsp_diagnostics_virtual_text_tidy|
       g:lsp_document_code_action_signs_enabled
 			     |g:lsp_document_code_action_signs_enabled|
       g:lsp_document_code_action_signs_delay
@@ -785,6 +787,20 @@ g:lsp_diagnostics_virtual_text_wrap       *g:lsp_diagnostics_virtual_text_wrap*
     Example: >
 	let g:lsp_diagnostics_virtual_text_wrap = "truncate"
 
+g:lsp_diagnostics_virtual_text_tidy
+		  *g:lsp_diagnostics_virtual_text_tidy*
+    Type: |Boolean|
+    Default: `0`
+
+    Determines whether or not to show only the diagnostic virtual text
+    with the highest severity per line. Requires
+    |g:lsp_diagnostics_virtual_text_enabled|.
+    This prevents unintentional newlines in the view inserted by diagnostics
+    messages.
+
+    Example: >
+	let g:lsp_diagnostics_virtual_text_tidy = 0
+
 g:lsp_document_code_action_signs_enabled
 				      *g:lsp_document_code_action_signs_enabled*
     Type: |Number|

plugin/lsp.vim

@@ -41,6 +41,7 @@ let g:lsp_diagnostics_virtual_text_prefix = get(g:, 'lsp_diagnostics_virtual_tex
 let g:lsp_diagnostics_virtual_text_align = get(g:, 'lsp_diagnostics_virtual_text_align', 'below')
 let g:lsp_diagnostics_virtual_text_wrap = get(g:, 'lsp_diagnostics_virtual_text_wrap', 'wrap')
 let g:lsp_diagnostics_virtual_text_padding_left = get(g:, 'lsp_diagnostics_virtual_text_padding_left', 1)
+let g:lsp_diagnostics_virtual_text_tidy = get(g:, 'lsp_diagnostics_virtual_text_tidy', 0)
 
 let g:lsp_document_code_action_signs_enabled = get(g:, 'lsp_document_code_action_signs_enabled', 1)
 let g:lsp_document_code_action_signs_delay = get(g:, 'lsp_document_code_action_signs_delay', 500)