Support wayland fractional scale protocol

The new protocol adds support for more native communication of
fractional scaling.

Everything in the wayland backend already existed only our fractional
scale was calculated implicitly through a combination of output size
guesswork for fullscreen windows.

This new protocol makes that explicit, providing a more robust solution
and a solution for non-fullscreen surfaces. The fallback code is still
left in place for now whilst compositors gain support.

src/video/wayland/SDL_waylandvideo.c

@@ -52,6 +52,7 @@
 #include "xdg-output-unstable-v1-client-protocol.h"
 #include "viewporter-client-protocol.h"
 #include "primary-selection-unstable-v1-client-protocol.h"
+#include "fractional-scale-v1-client-protocol.h"
 
 #ifdef HAVE_LIBDECOR_H
 #include <libdecor.h>
@@ -884,7 +885,8 @@ display_handle_global(void *data, struct wl_registry *registry, uint32_t id,
         Wayland_init_xdg_output(d);
     } else if (SDL_strcmp(interface, "wp_viewporter") == 0) {
         d->viewporter = wl_registry_bind(d->registry, id, &wp_viewporter_interface, 1);
-
+    } else if (SDL_strcmp(interface, "wp_fractional_scale_manager_v1") == 0) {
+        d->fractional_scale_manager = wl_registry_bind(d->registry, id, &wp_fractional_scale_manager_v1_interface, 1);
 #ifdef SDL_VIDEO_DRIVER_WAYLAND_QT_TOUCH
     } else if (SDL_strcmp(interface, "qt_touch_extension") == 0) {
         Wayland_touch_create(d, id);
@@ -1136,6 +1138,11 @@ Wayland_VideoCleanup(_THIS)
         data->primary_selection_device_manager = NULL;
     }
 
+    if (data->fractional_scale_manager) {
+        wp_fractional_scale_manager_v1_destroy(data->fractional_scale_manager);
+        data->fractional_scale_manager = NULL;
+    }
+
     if (data->compositor) {
         wl_compositor_destroy(data->compositor);
         data->compositor = NULL;

src/video/wayland/SDL_waylandvideo.h

@@ -75,6 +75,7 @@ typedef struct {
     struct zwp_text_input_manager_v3 *text_input_manager;
     struct zxdg_output_manager_v1 *xdg_output_manager;
     struct wp_viewporter *viewporter;
+    struct wp_fractional_scale_manager_v1 *fractional_scale_manager;
 
     EGLDisplay edpy;
     EGLContext context;

src/video/wayland/SDL_waylandwindow.c

@@ -38,6 +38,7 @@
 #include "idle-inhibit-unstable-v1-client-protocol.h"
 #include "xdg-activation-v1-client-protocol.h"
 #include "viewporter-client-protocol.h"
+#include "fractional-scale-v1-client-protocol.h"
 
 #ifdef HAVE_LIBDECOR_H
 #include <libdecor.h>
@@ -1104,7 +1105,10 @@ handle_surface_enter(void *data, struct wl_surface *surface,
 
     /* Update the scale factor after the move so that fullscreen outputs are updated. */
     Wayland_move_window(window->sdlwindow, driverdata);
+
+    if (!window->fractional_scale) {
         update_scale_factor(window);
+    }
 }
 
 static void
@@ -1142,7 +1146,9 @@ handle_surface_leave(void *data, struct wl_surface *surface,
                             window->outputs[window->num_outputs - 1]);
     }
 
+    if (!window->fractional_scale) {
         update_scale_factor(window);
+    }
 }
 
 static const struct wl_surface_listener surface_listener = {
@@ -1581,6 +1587,29 @@ Wayland_FlashWindow(_THIS, SDL_Window *window, SDL_FlashOperation operation)
     return 0;
 }
 
+void handle_preferred_scale_changed(void *data,
+                     struct wp_fractional_scale_v1 *wp_fractional_scale_v1,
+                     uint preferred_scale)
+{
+    SDL_WindowData *window = data;
+    float old_factor = window->scale_factor;
+    float new_factor = preferred_scale / 120.; /* 120 is a magic number defined in the spec as a common denominator*/
+
+    if (!(window->sdlwindow->flags & SDL_WINDOW_ALLOW_HIGHDPI)) {
+        /* Scale will always be 1, just ignore this */
+        return;
+    }
+
+    if (!FloatEqual(new_factor, old_factor)) {
+        Wayland_HandleResize(window->sdlwindow, window->sdlwindow->w, window->sdlwindow->h, new_factor);
+    }
+}
+
+static const struct wp_fractional_scale_v1_listener fractional_scale_listener = {
+    handle_preferred_scale_changed
+};
+
+
 #ifdef SDL_VIDEO_DRIVER_WAYLAND_QT_TOUCH
 static void SDLCALL
 QtExtendedSurface_OnHintChanged(void *userdata, const char *name,
@@ -1999,6 +2028,12 @@ int Wayland_CreateWindow(_THIS, SDL_Window *window)
         Wayland_input_lock_pointer(c->input);
     }
 
+    if (c->fractional_scale_manager) {
+        data->fractional_scale = wp_fractional_scale_manager_v1_get_fractional_scale(c->fractional_scale_manager, data->surface);
+        wp_fractional_scale_v1_add_listener(data->fractional_scale,
+                                         &fractional_scale_listener, data);
+    }
+
     /* Moved this call to ShowWindow: wl_surface_commit(data->surface); */
     WAYLAND_wl_display_flush(c->display);
 
@@ -2192,6 +2227,10 @@ void Wayland_DestroyWindow(_THIS, SDL_Window *window)
             wp_viewport_destroy(wind->draw_viewport);
         }
 
+        if (wind->fractional_scale) {
+            wp_fractional_scale_v1_destroy(wind->fractional_scale);
+        }
+
         SDL_free(wind->outputs);
 
         if (wind->gles_swap_frame_callback) {

src/video/wayland/SDL_waylandwindow.h

@@ -83,6 +83,7 @@ typedef struct {
     struct zwp_idle_inhibitor_v1 *idle_inhibitor;
     struct xdg_activation_token_v1 *activation_token;
     struct wp_viewport *draw_viewport;
+    struct wp_fractional_scale_v1 *fractional_scale;
 
     /* floating dimensions for restoring from maximized and fullscreen */
     int floating_width, floating_height;

wayland-protocols/fractional-scale-v1.xml

@@ -0,0 +1,102 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<protocol name="fractional_scale_v1">
+  <copyright>
+    Copyright © 2022 Kenny Levinsen
+
+    Permission is hereby granted, free of charge, to any person obtaining a
+    copy of this software and associated documentation files (the "Software"),
+    to deal in the Software without restriction, including without limitation
+    the rights to use, copy, modify, merge, publish, distribute, sublicense,
+    and/or sell copies of the Software, and to permit persons to whom the
+    Software is furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice (including the next
+    paragraph) shall be included in all copies or substantial portions of the
+    Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+    DEALINGS IN THE SOFTWARE.
+  </copyright>
+
+  <description summary="Protocol for requesting fractional surface scales">
+    This protocol allows a compositor to suggest for surfaces to render at
+    fractional scales.
+
+    A client can submit scaled content by utilizing wp_viewport. This is done by
+    creating a wp_viewport object for the surface and setting the destination
+    rectangle to the surface size before the scale factor is applied.
+
+    The buffer size is calculated by multiplying the surface size by the
+    intended scale.
+
+    The wl_surface buffer scale should remain set to 1.
+
+    If a surface has a surface-local size of 100 px by 50 px and wishes to
+    submit buffers with a scale of 1.5, then a buffer of 150px by 75 px should
+    be used and the wp_viewport destination rectangle should be 100 px by 50 px.
+
+    For toplevel surfaces, the size is rounded halfway away from zero. The
+    rounding algorithm for subsurface position and size is not defined.
+  </description>
+
+  <interface name="wp_fractional_scale_manager_v1" version="1">
+    <description summary="fractional surface scale information">
+      A global interface for requesting surfaces to use fractional scales.
+    </description>
+
+    <request name="destroy" type="destructor">
+      <description summary="unbind the fractional surface scale interface">
+        Informs the server that the client will not be using this protocol
+        object anymore. This does not affect any other objects,
+        wp_fractional_scale_v1 objects included.
+      </description>
+    </request>
+
+    <enum name="error">
+      <entry name="fractional_scale_exists" value="0"
+        summary="the surface already has a fractional_scale object associated"/>
+    </enum>
+
+    <request name="get_fractional_scale">
+      <description summary="extend surface interface for scale information">
+        Create an add-on object for the the wl_surface to let the compositor
+        request fractional scales. If the given wl_surface already has a
+        wp_fractional_scale_v1 object associated, the fractional_scale_exists
+        protocol error is raised.
+      </description>
+      <arg name="id" type="new_id" interface="wp_fractional_scale_v1"
+           summary="the new surface scale info interface id"/>
+      <arg name="surface" type="object" interface="wl_surface"
+           summary="the surface"/>
+    </request>
+  </interface>
+
+  <interface name="wp_fractional_scale_v1" version="1">
+    <description summary="fractional scale interface to a wl_surface">
+      An additional interface to a wl_surface object which allows the compositor
+      to inform the client of the preferred scale.
+    </description>
+
+    <request name="destroy" type="destructor">
+      <description summary="remove surface scale information for surface">
+        Destroy the fractional scale object. When this object is destroyed,
+        preferred_scale events will no longer be sent.
+      </description>
+    </request>
+
+    <event name="preferred_scale">
+      <description summary="notify of new preferred scale">
+        Notification of a new preferred scale for this surface that the
+        compositor suggests that the client should use.
+
+        The sent scale is the numerator of a fraction with a denominator of 120.
+      </description>
+      <arg name="scale" type="uint" summary="the new preferred scale"/>
+    </event>
+  </interface>
+</protocol>