wayland: use wl_compositor version 6 when available

terminal.c

@@ -2071,6 +2071,12 @@ term_fractional_scaling(const struct terminal *term)
     return term->wl->fractional_scale_manager != NULL && term->window->scale > 0.;
 }
 
+bool
+term_preferred_buffer_scale(const struct terminal *term)
+{
+    return term->wl->has_wl_compositor_v6;
+}
+
 bool
 term_update_scale(struct terminal *term)
 {
@@ -2081,6 +2087,9 @@ term_update_scale(struct terminal *term)
      * the scale in the following order:
      *
      *  - “preferred” scale, from the fractional-scale-v1 protocol
+     *  - "preferred" scale, from wl_compositor version 6.
+          NOTE: if the compositor advertises version 6 we must use 1.0
+          until wl_surface.preferred_buffer_scale is sent
      *  - scaling factor of output we most recently were mapped on
      *  - if we're not mapped, use the last known scaling factor
      *  - if we're not mapped, and we don't have a last known scaling
@@ -2090,13 +2099,15 @@ term_update_scale(struct terminal *term)
      */
     const float new_scale = (term_fractional_scaling(term)
         ? win->scale
-        : tll_length(win->on_outputs) > 0
+        : term_preferred_buffer_scale(term)
-            ? tll_back(win->on_outputs)->scale
+            ? win->preferred_buffer_scale
-            : term->scale_before_unmap > 0.
+            : tll_length(win->on_outputs) > 0
-                ? term->scale_before_unmap
+                ? tll_back(win->on_outputs)->scale
-                : tll_length(term->wl->monitors) > 0
+                : term->scale_before_unmap > 0.
-                    ? tll_front(term->wl->monitors).scale
+                    ? term->scale_before_unmap
-                    : 1.);
+                    : tll_length(term->wl->monitors) > 0
+                        ? tll_front(term->wl->monitors).scale
+                        : 1.);
 
     if (new_scale == term->scale)
         return false;

terminal.h

@@ -743,6 +743,7 @@ bool term_paste_data_to_slave(
     struct terminal *term, const void *data, size_t len);
 
 bool term_fractional_scaling(const struct terminal *term);
+bool term_preferred_buffer_scale(const struct terminal *term);
 bool term_update_scale(struct terminal *term);
 bool term_font_size_increase(struct terminal *term);
 bool term_font_size_decrease(struct terminal *term);

wayland.c

@@ -705,9 +705,37 @@ surface_leave(void *data, struct wl_surface *wl_surface,
     LOG_WARN("unmapped from unknown output");
 }
 
+#if defined(WL_SURFACE_PREFERRED_BUFFER_SCALE_SINCE_VERSION)
+static void
+surface_preferred_buffer_scale(void *data, struct wl_surface *surface,
+                               int32_t scale)
+{
+    struct wl_window *win = data;
+
+    if (win->preferred_buffer_scale == scale)
+        return;
+
+    LOG_DBG("wl_surface preferred scale: %d -> %d", win->preferred_buffer_scale, scale);
+
+    win->preferred_buffer_scale = scale;
+    update_term_for_output_change(win->term);
+}
+
+static void
+surface_preferred_buffer_transform(void *data, struct wl_surface *surface,
+                                   uint32_t transform)
+{
+
+}
+#endif
+
 static const struct wl_surface_listener surface_listener = {
     .enter = &surface_enter,
     .leave = &surface_leave,
+#if defined(WL_SURFACE_PREFERRED_BUFFER_SCALE_SINCE_VERSION)
+    .preferred_buffer_scale = &surface_preferred_buffer_scale,
+    .preferred_buffer_transform = &surface_preferred_buffer_transform,
+#endif
 };
 
 static void
@@ -1057,8 +1085,14 @@ handle_global(void *data, struct wl_registry *registry,
         if (!verify_iface_version(interface, version, required))
             return;
 
+#if defined (WL_SURFACE_PREFERRED_BUFFER_SCALE_SINCE_VERSION)
+        const uint32_t preferred = WL_SURFACE_PREFERRED_BUFFER_SCALE_SINCE_VERSION;
+        wayl->has_wl_compositor_v6 = version >= WL_SURFACE_PREFERRED_BUFFER_SCALE_SINCE_VERSION;
+#else
+        const uint32_t preferred = required;
+#endif
         wayl->compositor = wl_registry_bind(
-            wayl->registry, name, &wl_compositor_interface, required);
+            wayl->registry, name, &wl_compositor_interface, min(version, preferred));
     }
 
     else if (strcmp(interface, wl_subcompositor_interface.name) == 0) {
@@ -1700,6 +1734,10 @@ wayl_win_init(struct terminal *term, const char *token)
             win->fractional_scale, &fractional_scale_listener, win);
     }
 
+    if (wayl->has_wl_compositor_v6) {
+        win->preferred_buffer_scale = 1;
+    }
+
     win->xdg_surface = xdg_wm_base_get_xdg_surface(wayl->shell, win->surface.surf);
     xdg_surface_add_listener(win->xdg_surface, &xdg_surface_listener, win);
 
@@ -2020,8 +2058,11 @@ surface_scale_explicit_width_height(
         wp_viewport_set_destination(
             surf->viewport, roundf(width / scale), roundf(height / scale));
     } else {
-        LOG_DBG("scaling by a factor of %.2f using legacy mode "
+        const char *mode = term_preferred_buffer_scale(win->term)
-                "(width=%d, height=%d)", scale, width, height);
+            ? "wl_surface.preferred_buffer_scale"
+            : "legacy mode";
+        LOG_DBG("scaling by a factor of %.2f using %s "
+                "(width=%d, height=%d)" , scale, mode, width, height);
 
         xassert(scale == floorf(scale));
         const int iscale = (int)floorf(scale);

wayland.h

@@ -363,6 +363,7 @@ struct wl_window {
 
     bool unmapped;
     float scale;
+    int preferred_buffer_scale;
 
     struct zxdg_toplevel_decoration_v1 *xdg_toplevel_decoration;
 
@@ -429,6 +430,8 @@ struct wayland {
     struct wl_subcompositor *sub_compositor;
     struct wl_shm *shm;
 
+    bool has_wl_compositor_v6;
+
     struct zxdg_output_manager_v1 *xdg_output_manager;
 
     struct xdg_wm_base *shell;