fix cursor drift + tests

src/core/file_buffer.odin

@@ -527,9 +527,12 @@ move_cursor_down :: proc(buffer: ^FileBuffer, amount: int = 1, cursor: Maybe(^Cu
             break;
         }
     }
+    if it.hit_end {
+        return
+    }
 
     line_length := file_buffer_line_length(buffer, it.cursor.index);
-    if it.cursor.col < line_length && it.cursor.col < current_col {
+    if it.cursor.col < line_length-1 && it.cursor.col < current_col {
         for _ in iterate_file_buffer(&it) {
             if it.cursor.col >= line_length-1 || it.cursor.col >= current_col {
                 break;

src/tests/tests.odin

@@ -322,6 +322,49 @@ delete_across_slices :: proc(t: ^testing.T) {
     testing.expectf(t, contents == expected_text, "got '%v', expected '%v'", contents, expected_text)
 }
 
+@(test)
+move_down_next_line_has_shorter_length :: proc(t: ^testing.T) {
+    e := new_test_editor()
+    setup_empty_buffer(&e)
+
+    is_ctrl_pressed := false
+
+    buffer := &e.buffers[0]
+
+    run_text_insertion(&e, "012345678\n0")
+
+    // Move up to the first line
+    run_input_multiple(&e, press_key(.K), 1)
+
+    // Move to the end of the line
+    run_inputs(&e, []ArtificialInput{ press_key(.G), press_key(.L)})
+
+    // Move down to the second line
+    run_input_multiple(&e, press_key(.J), 1)
+
+    expect_line_col(t, buffer.cursor, 1, 0)
+    expect_cursor_index(t, buffer.cursor, 0, 10)
+}
+
+@(test)
+move_down_on_last_line :: proc(t: ^testing.T) {
+    e := new_test_editor()
+    setup_empty_buffer(&e)
+
+    is_ctrl_pressed := false
+
+    buffer := &e.buffers[0]
+
+    run_text_insertion(&e, "012345678")
+
+    // Try to move down
+    run_input_multiple(&e, press_key(.J), 1)
+
+    // Cursor should stay where it is
+    expect_line_col(t, buffer.cursor, 0, 8)
+    expect_cursor_index(t, buffer.cursor, 0, 8)
+}
+
 @(test)
 move_left_at_beginning_of_file :: proc(t: ^testing.T) {
     e := new_test_editor()