fix: remove unnecessary screen width rounding in ChangeScreenSize (#11)

## Summary
- Removes the Go-level `s.Width = s.Width - (s.Width % 8)` rounding in
`ChangeScreenSize` that was silently truncating requested widths (e.g.
390 → 384)
- Makes `XCreateScreenMode` return the actual dimensions that `libxcvt`
produced via in/out pointer parameters, so the Go caller uses the
correct width for `XSetScreenConfiguration`
- Fixes the mode name in `XCreateScreenModeInfo` to use `libxcvt`'s
actual output dimensions instead of the original request, preventing
name/geometry mismatches

## Context

The rounding was originally added to match `libxcvt`'s CVT convention of
producing widths as multiples of 8. However, CVT is a timing standard
for generating timing parameters for physical monitors — not a hard
requirement of X.Org or the Xorg dummy driver used for virtual displays.
The rounding made it impossible to use non-multiple-of-8 resolutions
like 390x844 for mobile viewports, even when valid modelines existed in
`xorg.conf`, because Neko would turn 390 into 384 before ever searching
for a mode.

Now the resolution flow is:
1. **Exact match** — if a pre-defined modeline exists in `xorg.conf` for
the exact requested dimensions (e.g. `390x844`),
`XSetScreenConfiguration` finds and uses it directly.
2. **Dynamic creation** — if no match, `XCreateScreenMode` dynamically
creates a new mode via `libxcvt` and reports the actual dimensions back
to the caller.

## Verified

Tested end-to-end in a headful `chromium-headful` container:
- Requested 390x844@25Hz via the kernel-images API
- xrandr confirmed display at exactly `390x844_25` (using a manually
crafted modeline in `xorg.conf` that bypasses CVT rounding)
- CDP verification confirmed `screen.width=390`, `screen.height=844`,
`innerWidth=390`, `innerHeight=807` (37px app-mode title bar)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Touches XRandR mode selection/creation and changes the
resolution-setting flow, which could affect display configuration
behavior across environments, but the change is localized to the Xorg
wrapper.
> 
> **Overview**
> `ChangeScreenSize` no longer truncates widths to a multiple of 8
before attempting to apply a screen mode; it now tries to set the exact
requested dimensions first and only creates a new mode if no existing
mode matches.
> 
> `XCreateScreenMode` is updated to take width/height as in/out pointers
so the caller receives libxcvt’s *actual* generated dimensions, and
`XCreateScreenModeInfo` now names modes using those actual dimensions to
avoid name/geometry mismatches.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
3f90ca6. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

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 +209,21 @@ 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 no existing mode matches, dynamically create one 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

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);