• VSCode Version: Version 1.36.1
• OS Version: macOS 10.14.5

Steps to Reproduce:

1. Create a file with a first line that is longer that 100 characters that matches a firstLine regexp of some language, where the regexp wants to match something at the end of the line.
   As an example the shellscript language defines the firstLine detection with:

"firstLine": "^#!.*\\b(bash|zsh|sh|tcsh).*|^#\\s*-\\*-[^*]*mode:\\s*shell-script[^*]*-\\*-",

which wants to match a -*- at the end of the line.
2. Open the file and observe that the language is not detected. Shortening the first line to no more than 100 characters, closing the line and opening it again, makes the language be detected.
E.g. a file name test-file containing the following text:

# -*- mode:shell-script; some-more:1; some-more:1; some-more:1; some-more:1; some-more7890123:12; -*-
#234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890
#        ^10       ^20       ^30       ^40       ^50       ^60       ^70       ^80       ^90       ^100

will not be detected as containing shellscript language. Changing the final "12" to "1" brings the length down to 100 and now it will be detected as shellscript.

Presumably this is because of the 100 at textEditorModel.ts.

For Emacs-style mode lines, like the ones shellscript tries to detect, contain file-specific "settings", so they can be quite long. At a minimum, the magic number 100 ought to be documented.