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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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36
backend/wayland/output.c
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@ -566,6 +566,12 @@ static struct wlr_wl_output_layer *get_or_create_output_layer(
&output_layer_addon_impl);
layer->surface = wl_compositor_create_surface(output->backend->compositor);
// Set buffer scale for layer surfaces too
if (output->backend->max_parent_scale > 1) {
wl_surface_set_buffer_scale(layer->surface, output->backend->max_parent_scale);
}
layer->subsurface = wl_subcompositor_get_subsurface(
output->backend->subcompositor, layer->surface, output->surface);
@ -1047,6 +1053,14 @@ static void xdg_surface_handle_configure(void *data,
output->requested_height = 0;
}
// If buffer scale is set, multiply dimensions to account for it
// The parent compositor sends logical dimensions, but we need physical buffer dimensions
int32_t scale = output->backend->max_parent_scale;
if (scale > 1) {
req_width *= scale;
req_height *= scale;
}
if (output->unmap_callback != NULL) {
return;
}
@ -1110,7 +1124,15 @@ static struct wlr_wl_output *output_create(struct wlr_wl_backend *backend,
struct wlr_output_state state;
wlr_output_state_init(&state);
wlr_output_state_set_custom_mode(&state, 1280, 720, 0);
// Scale initial dimensions if parent has HiDPI scale
int32_t width = 1280;
int32_t height = 720;
if (backend->max_parent_scale > 1) {
width *= backend->max_parent_scale;
height *= backend->max_parent_scale;
}
wlr_output_state_set_custom_mode(&state, width, height, 0);
wlr_output_init(wlr_output, &backend->backend, &output_impl,
backend->event_loop, &state);
@ -1118,6 +1140,12 @@ static struct wlr_wl_output *output_create(struct wlr_wl_backend *backend,
wlr_output->adaptive_sync_status = WLR_OUTPUT_ADAPTIVE_SYNC_ENABLED;
// Set output scale to match parent compositor so clients render at correct DPI
if (backend->max_parent_scale > 1) {
wlr_output->scale = (float)backend->max_parent_scale;
wlr_log(WLR_DEBUG, "Set output scale to %.1f to match parent", wlr_output->scale);
}
size_t output_num = ++last_output_num;
char name[64];
@ -1167,6 +1195,12 @@ struct wlr_output *wlr_wl_output_create(struct wlr_backend *wlr_backend) {
return NULL;
}
// Set buffer scale to match parent compositor's maximum scale
if (backend->max_parent_scale > 1) {
wl_surface_set_buffer_scale(surface, backend->max_parent_scale);
wlr_log(WLR_DEBUG, "Set output surface buffer scale to %d", backend->max_parent_scale);
}
struct wlr_wl_output *output = output_create(backend, surface);
if (output == NULL) {
wl_surface_destroy(surface);