From 17d510365286a509b01060471b794a98ccbe0409 Mon Sep 17 00:00:00 2001
From: Dan Egnor <egnor@ofb.net>
Date: Thu, 23 Jul 2026 12:34:19 -0700
Subject: [PATCH] Wayland: guess the origin (0,0) output for a fresh window's
 fractional scale

When there is no focused or recently-focused window to key off of (the
typical first-window-at-startup case), glfwGetWaylandCurrentMonitorFractionalScale
fell back to the registry-first output (monitors[0]). Registry/connector
order is semi-arbitrary and uncorrelated with where a compositor actually
places a new window, so on a multi-monitor setup with mixed fractional
scales the first window could be born at the wrong size.

Prefer the output at the logical origin (0, 0) instead: the compositor's
top-left/primary output is a better guess for initial placement than
registry order. Fall back to monitors[0] when no output reports (0, 0).

Since xdg_output geometry (including position) arrives asynchronously and
x/y read zero until it does, wait for every monitor's logical geometry
before trusting the reported positions, so an as-yet-unpositioned output
cannot spuriously match (0, 0).

Discussed in #10268.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VjwP6hQjA6GMuiM7Mf9VAx
---
 glfw/wl_monitor.c | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/glfw/wl_monitor.c b/glfw/wl_monitor.c
index cbd45b10a..d05f13455 100644
--- a/glfw/wl_monitor.c
+++ b/glfw/wl_monitor.c
@@ -351,8 +351,31 @@ GLFWAPI double glfwGetWaylandCurrentMonitorFractionalScale(void)
         if (window && window->wl.monitorsCount > 0)
             monitor = window->wl.monitors[0];
     }
-    if (!monitor)
-        monitor = _glfw.monitors[0];
+    if (!monitor) {
+        // No focused window to key off of (typically the first window at
+        // startup). Prefer the output at the logical origin (0, 0): the
+        // compositor's top-left/primary output is a better guess for where a
+        // new window will be placed than registry order. Fall back to the
+        // first output if no output reports position (0, 0).
+        //
+        // xdg_output geometry arrives asynchronously and x/y remain zero until
+        // it does, so wait for every monitor's logical geometry before trusting
+        // the reported positions (otherwise an as-yet-unpositioned output would
+        // spuriously match (0, 0)).
+        for (int i = 0; i < _glfw.monitorCount; i++) {
+            _GLFWmonitor *m = _glfw.monitors[i];
+            while (m->wl.xdg_output && m->wl.xdg_logical_width <= 0) {
+                if (wl_display_roundtrip(_glfw.wl.display) < 0) break;
+            }
+        }
+        for (int i = 0; i < _glfw.monitorCount; i++) {
+            _GLFWmonitor *m = _glfw.monitors[i];
+            if (m->wl.xdg_output && m->wl.xdg_logical_width > 0 &&
+                m->wl.x == 0 && m->wl.y == 0) { monitor = m; break; }
+        }
+        if (!monitor)
+            monitor = _glfw.monitors[0];
+    }
 
     if (!monitor->wl.xdg_output)
         return monitor->wl.scale > 0 ? (double)monitor->wl.scale : 1.0;