Address #4165: provide a highlight group for the detailed diagnostic popup

README.md

@@ -1770,6 +1770,10 @@ You can also style the line that has the warning/error with these groups:
 - `YcmWarningLine`, which falls back to group `SyntasticWarningLine` if it
   exists
 
+Finally, you can also style the popup for the detailed diagnostics (it is shown
+if `g:ycm_show_detailed_diag_in_popup` is set) using the group `YcmErrorPopup`,
+which falls back to `ErrorMsg`.
+
 Note that the line highlighting groups only work when the
 [`g:ycm_enable_diagnostic_signs`](#the-gycm_enable_diagnostic_signs-option)
 option is set. If you want highlighted lines but no signs in the Vim gutter,

autoload/youcompleteme.vim

@@ -512,6 +512,10 @@ function! s:SetUpSyntaxHighlighting()
           \ 'combine': 0,
           \ 'override': 1 } )
   endif
+
+  if !hlexists( 'YcmErrorPopup' )
+    highlight default link YcmErrorPopup ErrorMsg
+  endif
 endfunction
 
 

doc/youcompleteme.txt

@@ -2017,6 +2017,10 @@ You can also style the line that has the warning/error with these groups:
 - 'YcmWarningLine', which falls back to group 'SyntasticWarningLine' if it
   exists
 
+Finally, you can also style the popup for the detailed diagnostics (it is shown
+if |g:ycm_show_detailed_diag_in_popup| is set) using the group 'YcmErrorPopup',
+which falls back to |ErrorMsg|.
+
 Note that the line highlighting groups only work when the
 |g:ycm_enable_diagnostic_signs| option is set. If you want highlighted lines
 but no signs in the Vim gutter, set the 'signcolumn' option to 'no' in your

python/ycm/youcompleteme.py

@@ -849,7 +849,7 @@ def ShowDetailedDiagnostic( self, message_in_popup ):
           'maxwidth': available_columns,
           'close': 'click',
           'fixed': 0,
-          'highlight': 'ErrorMsg',
+          'highlight': 'YcmErrorPopup',
           'border': [ 1, 1, 1, 1 ],
           # Close when moving cursor
           'moved': 'expr',