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.

Now ChangeScreenSize follows a 3-step fallback:
1. Try the exact requested dimensions (e.g. 390x844)
2. If no match, round width up to the nearest multiple of 8 and retry
   (e.g. 392x844 — matches predefined modelines in xorg.conf)
3. If still no match, dynamically create a new mode via libxcvt

The C layer is also updated so XCreateScreenMode returns the actual
dimensions that libxcvt produced (via in/out pointer parameters), and
XCreateScreenModeInfo uses the actual libxcvt output dimensions for the
mode name so it matches the real mode geometry.

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
@@ -212,16 +209,33 @@ func ChangeScreenSize(s types.ScreenSize) (types.ScreenSize, error) {
 	// convert variables to C types
 	c_width, c_height, c_rate := C.int(s.Width), C.int(s.Height), C.short(s.Rate)
 
-	// if screen configuration already exists, just set it
+	// try to set the screen configuration with the exact requested dimensions
 	status := C.XSetScreenConfiguration(c_width, c_height, c_rate)
+
+	// if the exact dimensions don't match an existing mode, the width may not
+	// be a multiple of 8 (Xorg/CVT convention). Try rounding up to the nearest
+	// multiple of 8 before creating a new mode, since predefined modelines in
+	// xorg.conf typically use aligned widths (e.g. 390 -> 392).
+	if status != C.RRSetConfigSuccess {
+		alignedWidth := roundUpTo8(s.Width)
+		if alignedWidth != s.Width {
+			c_width = C.int(alignedWidth)
+			status = C.XSetScreenConfiguration(c_width, c_height, c_rate)
+		}
+	}
+
+	// if still no match, dynamically create a new mode via libxcvt
 	if status != C.RRSetConfigSuccess {
-		// create new screen configuration
-		C.XCreateScreenMode(c_width, c_height, c_rate)
+		C.XCreateScreenMode(&c_width, &c_height, c_rate)
 
 		// screen configuration should exist now, set it
 		status = C.XSetScreenConfiguration(c_width, c_height, c_rate)
 	}
 
+	// update s with the actual dimensions that were set
+	s.Width = int(c_width)
+	s.Height = int(c_height)
+
 	var err error
 
 	// if screen configuration was not set successfully, return error
@@ -237,6 +251,14 @@ func ChangeScreenSize(s types.ScreenSize) (types.ScreenSize, error) {
 	return s, err
 }
 
+// roundUpTo8 rounds n up to the nearest multiple of 8.
+func roundUpTo8(n int) int {
+	if r := n % 8; r != 0 {
+		return n + (8 - r)
+	}
+	return n
+}
+
 func GetScreenSize() types.ScreenSize {
 	mu.Lock()
 	defer mu.Unlock()

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);