fix: fullscreen surface oversized when --force-scale-factor is set (#329)

When --fullscreen and --force-scale-factor=N are combined, the Wayland
surface was created at native_width*N x native_height*N instead of the
native display resolution. For example, a 800x480 display with
--force-scale-factor=1.3 produced a 1040x624 surface, causing UI to
render partially off-screen.

Root cause: wl_output_listener.mode stored the native display pixels
directly in view_properties_.width/height for the fullscreen case, but
the rest of the codebase treats view_properties_ as logical DIP. The
subsequent multiplication by current_scale_ in CreateRenderSurface then
over-scaled the surface dimensions.

force_scale_factor is intended to adjust the Flutter engine's device
pixel ratio (DPR) so that UI elements appear larger, not to increase
the surface buffer beyond the display's native resolution. The fix:

1. wl_output_listener.mode: when force_scale_factor is set, store the
   display dimensions as logical DIP (width / current_scale_), consistent
   with the xdg_toplevel_listener.configure callback which already does
   this division correctly.

2. CreateRenderSurface: when force_scale_factor is set, use
   display_max_width_/height_ directly for the surface dimensions to
   avoid floating-point rounding when current_scale_ is non-integer.

The non-force_scale_factor path (auto-detected HiDPI from wl_output.scale)
is unchanged.

Fixes #329

Author: aki1770-del

src/flutter/shell/platform/linux_embedded/window/elinux_window_wayland.cc

@@ -682,8 +682,20 @@ const wl_output_listener ELinuxWindowWayland::kWlOutputListener = {
 
         if (self->view_properties_.view_mode ==
             FlutterDesktopViewMode::kFullscreen) {
-          self->view_properties_.width = width;
-          self->view_properties_.height = height;
+          if (self->view_properties_.force_scale_factor) {
+            // force_scale_factor adjusts the Flutter engine's device pixel
+            // ratio only; it does not scale the surface buffer. Store the
+            // display dimensions as logical DIP so that the invariant
+            // view_properties_.width * current_scale_ == native_px holds,
+            // consistent with xdg_toplevel_listener.configure.
+            self->view_properties_.width =
+                static_cast<int32_t>(width / self->current_scale_);
+            self->view_properties_.height =
+                static_cast<int32_t>(height / self->current_scale_);
+          } else {
+            self->view_properties_.width = width;
+            self->view_properties_.height = height;
+          }
           self->request_redraw_ = true;
         }
 
@@ -1382,8 +1394,17 @@ bool ELinuxWindowWayland::CreateRenderSurface(int32_t width_px,
   }
 
   if (view_properties_.view_mode == FlutterDesktopViewMode::kFullscreen) {
-    width_px = view_properties_.width * current_scale_;
-    height_px = view_properties_.height * current_scale_;
+    if (view_properties_.force_scale_factor &&
+        display_max_width_ > 0 && display_max_height_ > 0) {
+      // When force_scale_factor is active, the surface must be the native
+      // display resolution (display_max). Using display_max directly avoids
+      // floating-point rounding when current_scale_ is non-integer (e.g. 1.3).
+      width_px = display_max_width_;
+      height_px = display_max_height_;
+    } else {
+      width_px = view_properties_.width * current_scale_;
+      height_px = view_properties_.height * current_scale_;
+    }
   }
 
   ELINUX_LOG(TRACE) << "Created the Wayland surface: " << width_px << "x"