ADDED   .github/workflows/msys2-build.yml
Index: .github/workflows/msys2-build.yml
==================================================================
--- /dev/null
+++ .github/workflows/msys2-build.yml
@@ -0,0 +1,32 @@
+name: msys2
+
+on:
+  push:
+
+jobs:
+  msys2:
+    runs-on: windows-2025
+
+    defaults:
+      run:
+        shell: msys2 {0}
+        working-directory: unix
+
+    steps:
+      - name: Init MSYS
+        uses: msys2/setup-msys2@v2
+        with:
+          msystem: MSYS
+          install: git base-devel msys2-devel zlib-devel
+          update: true
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Build
+        run: |
+          ./configure -C --prefix=/usr --enable-64bit || { cat config.log; cat config.cache; exit 1; }
+          make
+      - name: Run Tests (ignoring test io-53.4.1 which hangs)
+        run: make test
+        env:
+          ERROR_ON_FAILURES: 1
+          TESTFLAGS: -skip io-53.4.1

Index: unix/Makefile.in
==================================================================
--- unix/Makefile.in
+++ unix/Makefile.in
@@ -810,13 +810,10 @@
 		 mv /tmp/macher_output ${LIB_FILE}; chmod u+x ${LIB_FILE}; \
 	    fi; \
 	fi
 
 ${STUB_LIB_FILE}: ${STUB_LIB_OBJS}
-	@if [ "x${LIB_FILE}" = "xcygtcl${MAJOR_VERSION}.${MINOR_VERSION}.dll" ] ; then \
-	    ( cd ${TOP_DIR}/win; ${MAKE} winextensions ); \
-	fi
 	rm -f $@
 	@MAKE_STUB_LIB@
 
 # Make target which outputs the list of the .o contained in the Tcl lib useful
 # to build a single big shared library containing Tcl and other extensions.

Index: unix/configure
==================================================================
--- unix/configure
+++ unix/configure
@@ -5891,19 +5891,10 @@
 printf "%s\n" "$ac_cv_cygwin" >&6; }
 	    if test "$ac_cv_cygwin" = "no"; then
 		as_fn_error $? "${CC} is not a cygwin compiler." "$LINENO" 5
 	    fi
 	    do64bit_ok=yes
-	    if test "x${SHARED_BUILD}" = "x1"; then
-		echo "running cd ../win; ${CONFIG_SHELL-/bin/sh} ./configure $ac_configure_args --enable-64bit --host=x86_64-w64-mingw32"
-		# The eval makes quoting arguments work.
-		if cd ../win; eval ${CONFIG_SHELL-/bin/sh} ./configure $ac_configure_args --enable-64bit --host=x86_64-w64-mingw32; cd ../unix
-		then :
-		else
-		    { echo "configure: error: configure failed for ../win" 1>&2; exit 1; }
-		fi
-	    fi
 	    ;;
 	dgux*)
 	    SHLIB_CFLAGS="-K PIC"
 	    SHLIB_LD='${CC} -G'
 	    SHLIB_LD_LIBS=""

Index: unix/tcl.m4
==================================================================
--- unix/tcl.m4
+++ unix/tcl.m4
@@ -1099,19 +1099,10 @@
 	    )
 	    if test "$ac_cv_cygwin" = "no"; then
 		AC_MSG_ERROR([${CC} is not a cygwin compiler.])
 	    fi
 	    do64bit_ok=yes
-	    if test "x${SHARED_BUILD}" = "x1"; then
-		echo "running cd ../win; ${CONFIG_SHELL-/bin/sh} ./configure $ac_configure_args --enable-64bit --host=x86_64-w64-mingw32"
-		# The eval makes quoting arguments work.
-		if cd ../win; eval ${CONFIG_SHELL-/bin/sh} ./configure $ac_configure_args --enable-64bit --host=x86_64-w64-mingw32; cd ../unix
-		then :
-		else
-		    { echo "configure: error: configure failed for ../win" 1>&2; exit 1; }
-		fi
-	    fi
 	    ;;
 	dgux*)
 	    SHLIB_CFLAGS="-K PIC"
 	    SHLIB_LD='${CC} -G'
 	    SHLIB_LD_LIBS=""