fix: Stop replxx aborting when navigating multi-line buffers

mgconsole assembles multi-line queries itself via the continuation prompt and
does not use replxx's in-buffer multiline editing, which is buggy in the pinned
release-0.0.4. history_previous() calls prev_newline_position(_pos - 1) without
the `_pos > 0` guard its sibling history_next() has; with a newline at buffer
position 0 (navigating up through a recalled multi-line history entry) it passes
-1 and trips an assertion that aborts the process. Separately, in-buffer
multiline redraws clear to end of screen and erase already-printed output.

Patch replxx (replxx-patches/, applied via the replxx-proj PATCH_COMMAND) to add
the missing bounds guard to history_previous. This fixes the abort for any
newline-bearing buffer, including multi-line entries decoded from the history
file.

Rebind Ctrl-J (line feed, 0x0A) to commit_line so a bare newline submits the
current line like Enter instead of feeding replxx's NEW_LINE action. A multi-line
paste then submits one physical line at a time, which is what GetQuery expects,
and keeps pasted or typed newlines out of the buffer so the redraw corruption
stays unreachable for the common paste case. It does not cover newlines that
arrive from history, which is why the bounds guard is the actual crash fix.

Author: Ignition

.gitattributes

@@ -0,0 +1,3 @@
+# git apply needs LF context lines to match an LF-checked-out source tree;
+# Windows autocrlf would otherwise rewrite these to CRLF and break the patch.
+*.patch text eol=lf

src/utils/CMakeLists.txt

@@ -9,10 +9,24 @@ else()
 endif()
 
 get_filename_component(REPLXX_LIB_PATH "${REPLXX_PREFIX}/${MG_INSTALL_LIB_DIR}/libreplxx${REPLXX_LIB_POSTFIX}.a" ABSOLUTE)
+
+# Local fixes applied on top of the pinned replxx tag.
+find_package(Git REQUIRED)
 ExternalProject_Add(replxx-proj
         PREFIX ${REPLXX_PREFIX}
         GIT_REPOSITORY https://github.com/AmokHuginnsson/replxx.git
         GIT_TAG release-0.0.4
+        # Force an LF checkout so the LF patch below applies. Windows/MSYS2 git
+        # defaults to core.autocrlf=true, which rewrites the sources with CRLF
+        # endings and makes the patch fail with "patch does not apply" (the
+        # trailing-CR context lines no longer match). --config persists into the
+        # cloned repo, so the update-step checkout stays LF too.
+        GIT_CONFIG core.autocrlf=false
+        # --3way makes this idempotent: the patch step chains off the git
+        # update step and can re-run, and --3way no-ops cleanly when the fix is
+        # already present (plain `git apply` would error "patch does not apply").
+        PATCH_COMMAND ${GIT_EXECUTABLE} apply --3way
+        "${CMAKE_CURRENT_SOURCE_DIR}/replxx-patches/0001-history_previous-bounds-check.patch"
         CMAKE_ARGS "-DCMAKE_INSTALL_PREFIX=<INSTALL_DIR>"
         "-DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}"
         "-DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}"

src/utils/replxx-patches/0001-history_previous-bounds-check.patch

@@ -0,0 +1,24 @@
+Add the missing bounds guard to ReplxxImpl::history_previous.
+
+When the edit buffer contains newlines and the cursor sits on a newline at
+position 0, history_previous indexed prev_newline_position(_pos - 1), that is
+prev_newline_position(-1), tripping the `pos_ >= 0` assertion and aborting the
+process. history_next already guards the symmetric case with
+`_pos > 0 ? ... : -1`; apply the same guard so navigating up through a
+multi-line buffer falls through to history_move instead of aborting.
+
+Not fixed in the pinned upstream tag.
+
+diff --git a/src/replxx_impl.cxx b/src/replxx_impl.cxx
+index 22ee748..24a63b8 100644
+--- a/src/replxx_impl.cxx
++++ b/src/replxx_impl.cxx
+@@ -1831,7 +1831,7 @@ Replxx::ACTION_RESULT Replxx::ReplxxImpl::history_previous( char32_t ) {
+ 		}
+ 		int prevNewlinePosition( prev_newline_position( _pos ) );
+ 		if ( prevNewlinePosition == _pos ) {
+-			prevNewlinePosition = prev_newline_position( _pos - 1 );
++			prevNewlinePosition = _pos > 0 ? prev_newline_position( _pos - 1 ) : -1;
+ 		}
+ 		if ( prevNewlinePosition < 0 ) {
+ 			break;

src/utils/utils.cpp

@@ -1324,6 +1324,18 @@ Replxx *InitAndSetupReplxx() {
   replxx_set_unique_history(replxx_instance, 1);
   replxx_set_completion_callback(replxx_instance, CompletionHook, nullptr);
 
+  // Treat a bare line feed (Ctrl-J / 0x0A) like Enter: commit the current line
+  // instead of feeding replxx's NEW_LINE action. mgconsole assembles multi-line
+  // queries itself via the continuation prompt, so a multi-line paste should
+  // submit one physical line at a time rather than accumulate in replxx's edit
+  // buffer, whose in-buffer multiline redraw clears to end of screen and erases
+  // already-printed output.
+  //
+  // This only keeps typed and pasted newlines out of the buffer. A recalled
+  // multi-line history entry still contains them, so replxx's multiline cursor
+  // navigation has to stay correct independently of this bind.
+  replxx_bind_key_internal(replxx_instance, REPLXX_KEY_CONTROL('J'), "commit_line");
+
   // ToDo(the-joksim):
   //   - syntax highlighting disabled for now - figure out a smarter way of
   //     picking the right colors depending on the user's terminal settings