openxr: submit dummy layer if nothing is visible

src/backend/openxr/lines.rs

@@ -7,6 +7,7 @@ use std::{
         atomic::{AtomicUsize, Ordering},
         Arc,
     },
+    usize,
 };
 
 use vulkano::{command_buffer::CommandBufferUsage, format::Format, image::view::ImageView};
@@ -152,6 +153,14 @@ impl LinePool {
 
         quads
     }
+
+    /// the number of lines that are waiting to be drawn
+    pub(super) fn num_pending(&self) -> usize {
+        self.lines
+            .values()
+            .filter(|l| l.maybe_line.is_some())
+            .count()
+    }
 }
 
 pub(super) struct Line {

src/backend/openxr/mod.rs

@@ -7,6 +7,7 @@ use std::{
     time::Duration,
 };
 
+use glam::Affine3A;
 use openxr as xr;
 use vulkano::{command_buffer::CommandBufferUsage, Handle, VulkanObject};
 
@@ -280,6 +281,17 @@ pub fn openxr_run(running: Arc<AtomicBool>) -> Result<(), BackendError> {
             };
         }
 
+        if layers.is_empty() && lines.num_pending() == 0 {
+            // HACK: we need to submit at least 1 layer, else the session hangs
+            lines.draw_from(
+                pointer_lines[0],
+                Affine3A::IDENTITY,
+                0.002,
+                0,
+                &app_state.input_state.hmd,
+            );
+        }
+
         for quad in lines.present_xr(&xr_state, &mut command_buffer) {
             layers.push((0.0, quad));
         }