Improve the main-file detection

autoload/vimtex/state.vim

@@ -475,23 +475,23 @@ endfunction
 function! s:file_is_main(file) abort " {{{1
   if !filereadable(a:file) | return 0 | endif
 
+  let l:preamble = vimtex#parser#preamble(a:file, {
+        \ 'inclusive' : 1,
+        \ 'root' : fnamemodify(a:file, ':p:h'),
+        \})
+
   " Check if a:file is a main file by looking for the \documentclass command,
   " but ignore the following:
-  "
-  "   \documentclass[...]{subfiles}
-  "   \documentclass[...]{standalone}
-  "
-  let l:lines = readfile(a:file, 0, 50)
+  " * \documentclass[...]{subfiles}
+  " * \documentclass[...]{standalone}
+  let l:lines = copy(l:preamble)
   call filter(l:lines, 'v:val =~# ''^\s*\\documentclass\_\s*[\[{]''')
   call filter(l:lines, 'v:val !~# ''{subfiles}''')
   call filter(l:lines, 'v:val !~# ''{standalone}''')
   if len(l:lines) == 0 | return 0 | endif
 
-  " A main file contains `\begin{document}`
-  let l:lines = vimtex#parser#preamble(a:file, {
-        \ 'inclusive' : 1,
-        \ 'root' : fnamemodify(a:file, ':p:h'),
-        \})
+  " A main file must also contain `\begin{document}`
+  let l:lines = copy(l:preamble)
   call filter(l:lines, 'v:val =~# ''^\s*\\begin\s*{document}''')
   return len(l:lines) > 0
 endfunction

doc/vimtex.txt

@@ -453,8 +453,8 @@ Recursive search~
   and parent directories that
 
   1. includes the present file (directly or indirectly),
-  2. has a `\documentclass` line near the top, and
-  3. contains `\begin{document}` (or includes a file that does).
+  2. has a `\documentclass` line near the top of its parsed content, and
+  3. contains `\begin{document}` in its parsed content.
 
   As long as a project follows these rules, the recursive search should work.
   Notice, though, that the first point is important and is where the recursive
@@ -463,23 +463,6 @@ Recursive search~
 
   Let's consider a common construct that will fail: >
 
-    % preamble.tex
-    \documentclass{article}
-    \usepackage{mypackages}
-    ...
-
-    % main.tex
-    \input{preamble}
-    \begin{document}
-    Hello world
-    \end{document}
-<
-  In this case, `main.tex` is _not_ recognized as a valid main file, because
-  it does not include `\documentclass`. Further, no other main file is found
-  that includes this file. Thus, VimTeX is not able to properly parse it.
-
-  Another example where the algorithm may fail: >
-
     path1/main.tex
     path2/chapter.tex
 <

test/test-get-main/test-included-preamble/main.tex

@@ -0,0 +1,3 @@
+\input{preamble}
+\begin{document}
+\end{document}

test/test-get-main/test-included-preamble/preamble.tex

@@ -0,0 +1 @@
+\documentclass{minimal}

test/test-get-main/test.vim

@@ -60,4 +60,9 @@ call vimtex#test#main('test-bib-alternate/references.bib', 'test-bib-alternate/m
 call vimtex#test#main('test-standalone/a/a.tex', 'test-standalone/main.tex')
 call vimtex#test#main('test-standalone/a/a.tex', 'test-standalone/a/a.tex', 1)
 
+" Test included preamble
+call vimtex#test#main(
+      \ './test-included-preamble/preamble.tex',
+      \ './test-included-preamble/main.tex')
+
 call vimtex#test#finished()