Accept negative window position

It seems to work on some window managers.

Fixes #1242 <https://github.com/Genymobile/scrcpy/issues/1242>

app/scrcpy.1

@@ -131,13 +131,13 @@ Set a custom window title.
 .BI "\-\-window\-x " value
 Set the initial window horizontal position.
 
-Default is -1 (automatic).\n
+Default is "auto".\n
 
 .TP
 .BI "\-\-window\-y " value
 Set the initial window vertical position.
 
-Default is -1 (automatic).\n
+Default is "auto".\n
 
 .TP
 .BI "\-\-window\-width " value

app/src/cli.c

@@ -1,5 +1,6 @@
 #include "cli.h"
 
+#include <assert.h>
 #include <getopt.h>
 #include <stdint.h>
 #include <unistd.h>
@@ -116,11 +117,11 @@ scrcpy_print_usage(const char *arg0) {
         "\n"
         "    --window-x value\n"
         "        Set the initial window horizontal position.\n"
-        "        Default is -1 (automatic).\n"
+        "        Default is \"auto\".\n"
         "\n"
         "    --window-y value\n"
         "        Set the initial window vertical position.\n"
-        "        Default is -1 (automatic).\n"
+        "        Default is \"auto\".\n"
         "\n"
         "    --window-width value\n"
         "        Set the initial window width.\n"
@@ -302,8 +303,16 @@ parse_lock_video_orientation(const char *s, int8_t *lock_video_orientation) {
 
 static bool
 parse_window_position(const char *s, int16_t *position) {
+    // special value for "auto"
+    static_assert(WINDOW_POSITION_UNDEFINED == -0x8000);
+
+    if (!strcmp(s, "auto")) {
+        *position = WINDOW_POSITION_UNDEFINED;
+        return true;
+    }
+
     long value;
-    bool ok = parse_integer_arg(s, &value, false, -1, 0x7FFF,
+    bool ok = parse_integer_arg(s, &value, false, -0x7FFF, 0x7FFF,
                                 "window position");
     if (!ok) {
         return false;

app/src/scrcpy.h

@@ -21,8 +21,8 @@ struct scrcpy_options {
     uint32_t bit_rate;
     uint16_t max_fps;
     int8_t lock_video_orientation;
-    int16_t window_x;
+    int16_t window_x; // WINDOW_POSITION_UNDEFINED for "auto"
-    int16_t window_y;
+    int16_t window_y; // WINDOW_POSITION_UNDEFINED for "auto"
     uint16_t window_width;
     uint16_t window_height;
     bool show_touches;
@@ -51,8 +51,8 @@ struct scrcpy_options {
     .bit_rate = DEFAULT_BIT_RATE, \
     .max_fps = 0, \
     .lock_video_orientation = DEFAULT_LOCK_VIDEO_ORIENTATION, \
-    .window_x = -1, \
+    .window_x = WINDOW_POSITION_UNDEFINED, \
-    .window_y = -1, \
+    .window_y = WINDOW_POSITION_UNDEFINED, \
     .window_width = 0, \
     .window_height = 0, \
     .show_touches = false, \

app/src/screen.c

@@ -186,8 +186,10 @@ screen_init_rendering(struct screen *screen, const char *window_title,
         window_flags |= SDL_WINDOW_BORDERLESS;
     }
 
-    int x = window_x != -1 ? window_x : (int) SDL_WINDOWPOS_UNDEFINED;
+    int x = window_x != WINDOW_POSITION_UNDEFINED
-    int y = window_y != -1 ? window_y : (int) SDL_WINDOWPOS_UNDEFINED;
+          ? window_x : (int) SDL_WINDOWPOS_UNDEFINED;
+    int y = window_y != WINDOW_POSITION_UNDEFINED
+          ? window_y : (int) SDL_WINDOWPOS_UNDEFINED;
     screen->window = SDL_CreateWindow(window_title, x, y,
                                       window_size.width, window_size.height,
                                       window_flags);

app/src/screen.h

@@ -8,6 +8,8 @@
 #include "config.h"
 #include "common.h"
 
+#define WINDOW_POSITION_UNDEFINED (-0x8000)
+
 struct video_buffer;
 
 struct screen {
@@ -53,6 +55,7 @@ void
 screen_init(struct screen *screen);
 
 // initialize screen, create window, renderer and texture (window is hidden)
+// window_x and window_y accept WINDOW_POSITION_UNDEFINED
 bool
 screen_init_rendering(struct screen *screen, const char *window_title,
                       struct size frame_size, bool always_on_top,