macOS: Fix incorrect text sizes calaculated when using an external display that is set to mirror the main display

Fix #1056

docs/changelog.rst

@@ -32,6 +32,9 @@ Changelog
 - macOS: Fix rendering frames-per-second very low when processing
   large amounts of input in small chunks (:pull:`1082`)
 
+- macOS: Fix incorrect text sizes calaculated when using an external display
+  that is set to mirror the main display (:iss:`1056`)
+
 - Linux: Fix match rules used as aliases in Fontconfig configuration not being
   respected (:iss:`1085`)
 

kitty/glfw.c

@@ -343,6 +343,9 @@ get_window_dpi(GLFWwindow *w, double *x, double *y) {
     if (monitor == NULL) { PyErr_Print(); monitor = glfwGetPrimaryMonitor(); }
     float xscale = 1, yscale = 1;
     if (monitor) glfwGetMonitorContentScale(monitor, &xscale, &yscale);
+    if (!xscale || !yscale) glfwGetMonitorContentScale(glfwGetPrimaryMonitor(), &xscale, &yscale);
+    if (!xscale) xscale = 1.0;
+    if (!yscale) yscale = 1.0;
 #ifdef __APPLE__
     double factor = 72.0;
 #else