fix: remove unnecessary screen width rounding in ChangeScreenSize

The Go-level `s.Width = s.Width - (s.Width % 8)` rounding was silently
truncating requested widths (e.g. 390 → 384) before attempting to find
or create a screen mode. This made it impossible to use resolutions like
392x844 for mobile viewports even when valid modelines existed.

The rounding was originally added to match libxcvt's CVT convention of
producing widths as multiples of 8, but this is a timing standard for
physical monitors — not a constraint of the Xorg dummy driver used for
virtual displays.

Instead of pre-rounding in Go, we now:
- Pass the requested width directly to XSetScreenConfiguration
- Make XCreateScreenMode return the actual dimensions that libxcvt
  produced (via in/out pointer parameters)
- Use the actual libxcvt output dimensions for the mode name so it
  matches the real mode geometry
- Update the returned ScreenSize with the actual created dimensions

This way callers get accurate feedback about what resolution was set,
and existing modes with non-rounded widths (from xorg.conf) are found
correctly.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: hiroTamada

server/pkg/xorg/xorg.c

@@ -302,12 +302,18 @@ void XGetScreenConfigurations() {
 }
 
 // Inspired by https://github.com/raboof/xrandr/blob/master/xrandr.c
-void XCreateScreenMode(int width, int height, short rate) {
+// width and height are in/out: on return they contain the actual dimensions
+// of the created mode (libxcvt may round width to a multiple of 8).
+void XCreateScreenMode(int *width, int *height, short rate) {
   Display *display = getXDisplay();
   Window root = DefaultRootWindow(display);
 
   // create new mode info
-  XRRModeInfo *mode_info = XCreateScreenModeInfo(width, height, rate);
+  XRRModeInfo *mode_info = XCreateScreenModeInfo(*width, *height, rate);
+
+  // write back the actual dimensions that were created
+  *width = mode_info->width;
+  *height = mode_info->height;
 
   // create new mode
   RRMode mode = XRRCreateMode(display, root, mode_info);
@@ -325,16 +331,19 @@ void XCreateScreenMode(int width, int height, short rate) {
 
 // Inspired by https://fossies.org/linux/xwayland/hw/xwayland/xwayland-cvt.c
 XRRModeInfo *XCreateScreenModeInfo(int hdisplay, int vdisplay, short vrefresh) {
-  char name[128];
-  snprintf(name, sizeof name, "%dx%d_%d", hdisplay, vdisplay, vrefresh);
-  XRRModeInfo *modeinfo = XRRAllocModeInfo(name, strlen(name));
-
 #ifdef _LIBCVT_H_
   struct libxcvt_mode_info *mode_info;
 
   // get screen mode from libxcvt, if available
+  // NOTE: libxcvt may round hdisplay up to a multiple of 8 (CVT convention).
+  // We use the actual output dimensions for both the mode name and geometry
+  // so that they are consistent.
   mode_info = libxcvt_gen_mode_info(hdisplay, vdisplay, vrefresh, false, false);
 
+  char name[128];
+  snprintf(name, sizeof name, "%dx%d_%d", mode_info->hdisplay, mode_info->vdisplay, vrefresh);
+  XRRModeInfo *modeinfo = XRRAllocModeInfo(name, strlen(name));
+
   modeinfo->width      = mode_info->hdisplay;
   modeinfo->height     = mode_info->vdisplay;
   modeinfo->dotClock   = mode_info->dot_clock * 1000;
@@ -349,6 +358,10 @@ XRRModeInfo *XCreateScreenModeInfo(int hdisplay, int vdisplay, short vrefresh) {
   free(mode_info);
 #else
   // fallback to a simple mode without refresh rate
+  char name[128];
+  snprintf(name, sizeof name, "%dx%d_%d", hdisplay, vdisplay, vrefresh);
+  XRRModeInfo *modeinfo = XRRAllocModeInfo(name, strlen(name));
+
   modeinfo->width = hdisplay;
   modeinfo->height = vdisplay;
 #endif

server/pkg/xorg/xorg.go

@@ -201,9 +201,6 @@ func ChangeScreenSize(s types.ScreenSize) (types.ScreenSize, error) {
 	mu.Lock()
 	defer mu.Unlock()
 
-	// round width to 8, because of Xorg
-	s.Width = s.Width - (s.Width % 8)
-
 	// if rate is 0, set it to 60
 	if s.Rate == 0 {
 		s.Rate = 60
@@ -215,8 +212,14 @@ func ChangeScreenSize(s types.ScreenSize) (types.ScreenSize, error) {
 	// if screen configuration already exists, just set it
 	status := C.XSetScreenConfiguration(c_width, c_height, c_rate)
 	if status != C.RRSetConfigSuccess {
-		// create new screen configuration
-		C.XCreateScreenMode(c_width, c_height, c_rate)
+		// create new screen configuration; XCreateScreenMode may adjust
+		// width/height (e.g. libxcvt rounds width to a multiple of 8) so
+		// we use in/out parameters to get the actual dimensions back.
+		C.XCreateScreenMode(&c_width, &c_height, c_rate)
+
+		// update s with the actual dimensions created
+		s.Width = int(c_width)
+		s.Height = int(c_height)
 
 		// screen configuration should exist now, set it
 		status = C.XSetScreenConfiguration(c_width, c_height, c_rate)

server/pkg/xorg/xorg.h

@@ -39,7 +39,7 @@ void XKey(KeySym keysym, int down);
 Status XSetScreenConfiguration(int width, int height, short rate);
 void XGetScreenConfiguration(int *width, int *height, short *rate);
 void XGetScreenConfigurations();
-void XCreateScreenMode(int width, int height, short rate);
+void XCreateScreenMode(int *width, int *height, short rate);
 XRRModeInfo *XCreateScreenModeInfo(int hdisplay, int vdisplay, short vrefresh);
 
 void XSetKeyboardModifier(unsigned char mod, int on);