[GTK] Use primary monitor for device zoom on multi-monitor setups

Display._getDeviceZoom(long) ignored its monitor argument and always
queried the monitor at virtual coordinate (0,0), so every monitor
returned by getMonitors() reported the (0,0) monitor's scale factor on
X11/Xwayland. Device.getDeviceZoom() did the same on GTK3 for the
initial Display zoom.

On mixed-DPI multi-monitor setups this caused shells opened on a
non-(0,0) monitor to be sized for the wrong scale: e.g. a HiDPI laptop
panel sitting at the top-left of the virtual screen made shells on a
standard-density secondary monitor render at 2x.

Honor the passed monitor handle in _getDeviceZoom and pick the GDK
primary monitor for the initial zoom on GTK3, falling back to the
(0,0) monitor only when no primary is available (e.g. on Wayland).

Fixes #3273

Author: vogella

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/graphics/Device.java

@@ -23,6 +23,7 @@
 import org.eclipse.swt.internal.gtk.*;
 import org.eclipse.swt.internal.gtk3.*;
 import org.eclipse.swt.internal.gtk4.*;
+import org.eclipse.swt.widgets.*;
 
 /**
  * This class is the abstract superclass of all device objects,
@@ -1107,7 +1108,7 @@ protected int getDeviceZoom() {
 		long surface = GTK4.gtk_native_get_surface(GTK4.gtk_widget_get_native(shellHandle));
 		monitor = GDK.gdk_display_get_monitor_at_surface(display, surface);
 	} else {
-		monitor = GDK.gdk_display_get_monitor_at_point(display, 0, 0);
+		monitor = Display.getPrimaryMonitor(display);
 	}
 
 	// GDK can return null monitor in some cases thus play safe

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/Display.java

@@ -6256,19 +6256,36 @@ long gdk_device_get_surface_at_position (double[] win_x, double[] win_y) {
 	return GDK.gdk_device_get_surface_at_position (device, win_x, win_y);
 }
 
-static int _getDeviceZoom (long monitor_num) {
+static int _getDeviceZoom (long monitor) {
 	/*
 	 * We can hard-code 96 as gdk_screen_get_resolution will always return -1
 	 * if gdk_screen_set_resolution has not been called.
 	 */
 	int dpi = 96;
-	long display = GDK.gdk_display_get_default();
-	long monitor = GDK.gdk_display_get_monitor_at_point(display, 0, 0);
-	int scale = GDK.gdk_monitor_get_scale_factor(monitor);
-	dpi = dpi * scale;
+	if (monitor == 0) {
+		monitor = getPrimaryMonitor(GDK.gdk_display_get_default());
+	}
+	if (monitor != 0) {
+		int scale = GDK.gdk_monitor_get_scale_factor(monitor);
+		dpi = dpi * scale;
+	}
 	return DPIUtil.mapDPIToZoom (dpi);
 }
 
+/**
+ * Returns the GDK primary monitor handle, falling back to the monitor at
+ * virtual coordinate (0,0) when no primary monitor is reported (e.g. on Wayland).
+ *
+ * @noreference This method is not intended to be referenced by clients.
+ */
+public static long getPrimaryMonitor(long display) {
+	long monitor = GDK.gdk_display_get_primary_monitor(display);
+	if (monitor == 0) {
+		monitor = GDK.gdk_display_get_monitor_at_point(display, 0, 0);
+	}
+	return monitor;
+}
+
 static boolean isActivateShellOnForceFocus() {
 	return "true".equals(System.getProperty("org.eclipse.swt.internal.activateShellOnForceFocus", "true")); //$NON-NLS-1$
 }