replicate dwm behavior for snap resize

Enables the protocol documented at
https://wayland.app/protocols/ext-data-control-v1

It is the upstreamed version of the old wlr_data_control.

It is used e.g. by mpv to read and write the clipboard.

README.md

@@ -1,13 +1,5 @@
 # dwl - dwm for Wayland
 
-2025-08-16:  
-dwl IS CURRENTLY UN-MAINTAINED.  
-AT THE PRESENT TIME, I (@fauxmight) DO NOT HAVE  
-THE TIME OR CAPACITY TO KEEP UP WITH [wlroots] CHANGES.  
-IF YOU ARE INTERESTED IN TAKING ON LEAD DEVELOPER RESPONSIBILITIES,  
-SEE ISSUE [#1166](https://codeberg.org/dwl/dwl/issues/1166).
----
-
 Join us on our IRC channel: [#dwl on Libera Chat]  
 Or on the community-maintained [Discord server].
 

config.def.h

@@ -7,6 +7,7 @@
 static const int sloppyfocus               = 1;  /* focus follows mouse */
 static const int bypass_surface_visibility = 0;  /* 1 means idle inhibitors will disable idle tracking even if it's surface isn't visible  */
 static const unsigned int borderpx         = 1;  /* border pixel of windows */
+static const unsigned int snap             = 32; /* snap pixel */
 static const float rootcolor[]             = COLOR(0x222222ff);
 static const float bordercolor[]           = COLOR(0x444444ff);
 static const float focuscolor[]            = COLOR(0x005577ff);

dwl.c

@@ -24,6 +24,7 @@
 #include <wlr/types/wlr_data_device.h>
 #include <wlr/types/wlr_drm.h>
 #include <wlr/types/wlr_export_dmabuf_v1.h>
+#include <wlr/types/wlr_ext_data_control_v1.h>
 #include <wlr/types/wlr_fractional_scale_v1.h>
 #include <wlr/types/wlr_gamma_control_v1.h>
 #include <wlr/types/wlr_idle_inhibit_v1.h>
@@ -1862,6 +1863,7 @@ void
 motionnotify(uint32_t time, struct wlr_input_device *device, double dx, double dy,
 		double dx_unaccel, double dy_unaccel)
 {
+	int nx, ny;
 	double sx = 0, sy = 0, sx_confined, sy_confined;
 	Client *c = NULL, *w = NULL;
 	LayerSurface *l = NULL;
@@ -1919,7 +1921,17 @@ motionnotify(uint32_t time, struct wlr_input_device *device, double dx, double d
 	/* If we are currently grabbing the mouse, handle and return */
 	if (cursor_mode == CurMove) {
 		/* Move the grabbed client to the new position. */
-		resize(grabc, (struct wlr_box){.x = (int)round(cursor->x) - grabcx, .y = (int)round(cursor->y) - grabcy,
+		nx = (int)round(cursor->x) - grabcx;
+		ny = (int)round(cursor->y) - grabcy;
+		if (abs(selmon->w.x - nx) < (int)snap)
+			nx = selmon->w.x;
+		else if (abs((selmon->w.x + selmon->w.width) - (nx + grabc->geom.width)) < (int)snap)
+			nx = selmon->w.x + selmon->w.width - grabc->geom.width;
+		if (abs(selmon->w.y - ny) < (int)snap)
+			ny = selmon->w.y;
+		else if (abs((selmon->w.y + selmon->w.height) - (ny + grabc->geom.height)) < (int)snap)
+			ny = selmon->w.y + selmon->w.height - grabc->geom.height;
+		resize(grabc, (struct wlr_box){.x = nx, .y = ny,
 			.width = grabc->geom.width, .height = grabc->geom.height}, 1);
 		return;
 	} else if (cursor_mode == CurResize) {
@@ -2518,6 +2530,7 @@ setup(void)
 	wlr_export_dmabuf_manager_v1_create(dpy);
 	wlr_screencopy_manager_v1_create(dpy);
 	wlr_data_control_manager_v1_create(dpy);
+	wlr_ext_data_control_manager_v1_create(dpy, 1);
 	wlr_primary_selection_v1_device_manager_create(dpy);
 	wlr_viewporter_create(dpy);
 	wlr_single_pixel_buffer_manager_v1_create(dpy);