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backend/wayland: Fix HiDPI support for nested compositors

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When running a wlroots-based compositor nested under another Wayland
compositor with HiDPI scaling, the nested window appears blurry and
incorrectly sized. This is because the Wayland backend does not:

1. Detect and track the parent compositor's output scale
2. Set wl_surface_set_buffer_scale() to match the parent's scale
3. Multiply output dimensions by the scale factor when handling
   configure events

This patch fixes all three issues by:

- Adding wlr_wl_parent_output structure to track parent outputs
- Listening to wl_output.scale events from the parent compositor
- Setting buffer_scale on all backend surfaces (output and layers)
- Scaling output dimensions to account for buffer scale
- Setting wlr_output.scale so nested clients render at correct DPI

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/854
This commit is contained in:
Anatolii Smolianinov 2025-10-30 11:13:32 +01:00
parent 604fcdb1db
commit f758cd1115
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GPG key ID: 2DFC2826A71FE8A0
3 changed files with 134 additions and 1 deletions
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88
backend/wayland/backend.c
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@ -87,6 +87,48 @@ static int dispatch_events(int fd, uint32_t mask, void *data) {
return count;
}
static void output_handle_geometry(void *data, struct wl_output *wl_output,
int32_t x, int32_t y, int32_t phys_width, int32_t phys_height,
int32_t subpixel, const char *make, const char *model,
int32_t transform) {
// We don't care about geometry for now
}
static void output_handle_mode(void *data, struct wl_output *wl_output,
uint32_t flags, int32_t width, int32_t height, int32_t refresh) {
// We don't care about mode for now
}
static void output_handle_done(void *data, struct wl_output *wl_output) {
// Nothing to do
}
static void output_handle_scale(void *data, struct wl_output *wl_output,
int32_t factor) {
struct wlr_wl_parent_output *output = data;
output->scale = factor;
wlr_log(WLR_DEBUG, "Parent output scale changed to %d", factor);
// Update all backend outputs to use the new maximum scale
struct wlr_wl_backend *backend = output->backend;
int32_t max_scale = 1;
struct wlr_wl_parent_output *parent;
wl_list_for_each(parent, &backend->parent_outputs, link) {
if (parent->scale > max_scale) {
max_scale = parent->scale;
}
}
backend->max_parent_scale = max_scale;
}
static const struct wl_output_listener output_listener = {
.geometry = output_handle_geometry,
.mode = output_handle_mode,
.done = output_handle_done,
.scale = output_handle_scale,
};
static void xdg_wm_base_handle_ping(void *data,
struct xdg_wm_base *base, uint32_t serial) {
xdg_wm_base_pong(base, serial);
@ -419,9 +461,30 @@ static void registry_global(void *data, struct wl_registry *registry,
} else if (strcmp(iface, wp_linux_drm_syncobj_manager_v1_interface.name) == 0) {
wl->drm_syncobj_manager_v1 = wl_registry_bind(registry, name,
&wp_linux_drm_syncobj_manager_v1_interface, 1);
} else if (strcmp(iface, wl_output_interface.name) == 0) {
struct wlr_wl_parent_output *output = calloc(1, sizeof(*output));
if (output == NULL) {
wlr_log_errno(WLR_ERROR, "Failed to allocate parent output");
return;
}
output->backend = wl;
output->wl_output = wl_registry_bind(registry, name,
&wl_output_interface, 2); // version 2 for scale support
output->scale = 1;
output->global_name = name;
wl_output_add_listener(output->wl_output, &output_listener, output);
wl_list_insert(&wl->parent_outputs, &output->link);
wlr_log(WLR_DEBUG, "Bound to parent wl_output");
}
}
static void parent_output_destroy(struct wlr_wl_parent_output *output) {
wl_list_remove(&output->link);
wl_output_destroy(output->wl_output);
free(output);
}
static void registry_global_remove(void *data, struct wl_registry *registry,
uint32_t name) {
struct wlr_wl_backend *wl = data;
@ -433,6 +496,24 @@ static void registry_global_remove(void *data, struct wl_registry *registry,
break;
}
}
struct wlr_wl_parent_output *output, *tmp;
wl_list_for_each_safe(output, tmp, &wl->parent_outputs, link) {
if (output->global_name == name) {
parent_output_destroy(output);
// Recalculate max scale
int32_t max_scale = 1;
struct wlr_wl_parent_output *parent;
wl_list_for_each(parent, &wl->parent_outputs, link) {
if (parent->scale > max_scale) {
max_scale = parent->scale;
}
}
wl->max_parent_scale = max_scale;
break;
}
}
}
static const struct wl_registry_listener registry_listener = {
@ -485,6 +566,11 @@ static void backend_destroy(struct wlr_backend *backend) {
wlr_output_destroy(&output->wlr_output);
}
struct wlr_wl_parent_output *parent_output, *tmp_parent_output;
wl_list_for_each_safe(parent_output, tmp_parent_output, &wl->parent_outputs, link) {
parent_output_destroy(parent_output);
}
// Avoid using wl_list_for_each_safe() here: destroying a buffer may
// have the side-effect of destroying the next one in the list
while (!wl_list_empty(&wl->buffers)) {
@ -603,6 +689,8 @@ struct wlr_backend *wlr_wl_backend_create(struct wl_event_loop *loop,
wl_list_init(&wl->seats);
wl_list_init(&wl->buffers);
wl_list_init(&wl->drm_syncobj_timelines);
wl_list_init(&wl->parent_outputs);
wl->max_parent_scale = 1;
if (remote_display != NULL) {
wl->remote_display = remote_display;