From bbf3cf00316d162dc19a65c4ef302de3d70592f1 Mon Sep 17 00:00:00 2001
From: T-Gander <thomas@silverhat.ca>
Date: Wed, 19 Aug 2026 00:11:34 -0600
Subject: [PATCH] fix(linux/kms): count CRTCs not planes when assigning monitor
 index

kms_display_names() and display_t::init() each independently counted
every active, non-cursor plane as one "monitor" when building/matching
the display index. A CRTC with more than one simultaneously-active
plane (e.g. gamescope's base + overlay layers) was counted twice by
both passes, throwing their indices out of sync with each other and
producing duplicate/misnumbered display names.

Track counted CRTCs per card in both passes so a CRTC contributes to
the index exactly once, regardless of how many active planes it has.
---
 src/platform/linux/kmsgrab.cpp | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/src/platform/linux/kmsgrab.cpp b/src/platform/linux/kmsgrab.cpp
index fdf10a92..05c972a1 100644
--- a/src/platform/linux/kmsgrab.cpp
+++ b/src/platform/linux/kmsgrab.cpp
@@ -7,6 +7,7 @@
 #include <fcntl.h>
 #include <filesystem>
 #include <ranges>
+#include <set>
 #include <thread>
 #include <unistd.h>
 
@@ -932,6 +933,7 @@ namespace platf {
             continue;
           }
 
+          std::set<std::uint32_t> counted_crtcs;
           auto end = std::end(card);
           for (auto plane = std::begin(card); plane != end; ++plane) {
             // Skip unused planes
@@ -943,8 +945,15 @@ namespace platf {
               continue;
             }
 
+            // A CRTC can have more than one simultaneously-active plane (e.g. gamescope's
+            // base + overlay layers). Count each CRTC once so this matches kms_display_names().
+            if (counted_crtcs.count(plane->crtc_id)) {
+              continue;
+            }
+
             if (monitor != monitor_index) {
               ++monitor;
+              counted_crtcs.insert(plane->crtc_id);
               continue;
             }
 
@@ -2075,6 +2084,7 @@ namespace platf {
       }
 
       auto crtc_to_monitor = kms::map_crtc_to_monitor(card.monitors(conn_type_count));
+      std::set<std::uint32_t> counted_crtcs;
 
       auto end = std::end(card);
       for (auto plane = std::begin(card); plane != end; ++plane) {
@@ -2087,6 +2097,12 @@ namespace platf {
           continue;
         }
 
+        // A CRTC can have more than one simultaneously-active plane (e.g. gamescope's
+        // base + overlay layers). Count each CRTC once, not once per active plane.
+        if (counted_crtcs.count(plane->crtc_id)) {
+          continue;
+        }
+
         auto fb = card.fb(plane.get());
         if (!fb) {
           BOOST_LOG(error) << "Couldn't get drm fb for plane ["sv << plane->fb_id << "]: "sv << strerror(errno);
@@ -2126,6 +2142,7 @@ namespace platf {
         kms::print(plane.get(), fb.get(), crtc.get());
         display_names.emplace_back(std::format("{}-{}", drmModeGetConnectorTypeName(it->second.type), it->second.index));
         count++;
+        counted_crtcs.insert(plane->crtc_id);
       }
 
       cds.emplace_back(kms::card_descriptor_t {