Warn on questionable emergencystretch values.

CHANGELOG.md

@@ -9,6 +9,10 @@ Changelog
 
 All notable changes to lua-widow-control will be listed here, in reverse chronological order. **Changes listed in bold** are important changes: they either remove options or commands, or may change the location of page breaks.
 
+## Unreleased
+
+- Warn on questionable `emergencystretch` values.
+
 ## [v3.0.1 (2024-03-11)](https://github.com/gucci-on-fleek/lua-widow-control/releases/tag/release-1d224f39e2cadf74e5fb6a5d2dcd42c3fdaf2002)
 
 _Final release present in TeX Live 2023_

source/lua-widow-control.sty

@@ -63,16 +63,33 @@
         \c_right_brace_str\ instead.
     }
 
+\msg_new:nnn
+    { \c__lwc_name_str }
+    { dubious-emergencystretch }
+    {
+        Questionable~ `emergencystretch'~ value. \\
+        `emergencystretch'~ values~ greater~ than~ \c_backslash_str textwidth~
+        generally~ give~ poor~ results.~ Use~ at~ your~ own~ risk.~
+        (Current~ value:~ \dim_use:N \g__lwc_emergencystretch_dim)
+    }
+
 % Don't let the user proceed unless they are using \LuaTeX{}.
 \sys_if_engine_luatex:F {
     \msg_critical:nn { \c__lwc_name_str } { no-luatex }
 }
 
 % Define (most of) the keys
 \cs_generate_variant:Nn \keys_define:nn { Vn }
+\dim_new:N \g__lwc_emergencystretch_dim
 
 \keys_define:Vn { \c__lwc_name_str } {
-    emergencystretch .dim_gset:N       = \g__lwc_emergencystretch_dim,
+    emergencystretch .code:n           = {
+        \dim_gset:Nn \g__lwc_emergencystretch_dim { #1 }
+        \dim_compare:nNnT { #1 } > { \dim_max:nn { \textwidth } { 6in } }
+        {
+            \msg_warning:nn { \c__lwc_name_str } { dubious-emergencystretch }
+        }
+    },
     emergencystretch .value_required:n = true,
     emergencystretch .initial:x        = \dim_max:nn { 3em } { 30pt },