iris: a one-layer glyph atlas is a GL_TEXTURE_2D, so every glyph drew as a box

The emulator was blamed for two days for what is iris's own defect on any
GL adapter. `GpuTextures::new` created the atlas `texture_2d_array` with
one layer; wgpu-hal picks the GL target from the descriptor alone
(`gles::Texture::get_info_from_desc`, `(false, 1) => TEXTURE_2D`), so the
shader's `sampler2DArray` was handed a `GL_TEXTURE_2D`, the unit was
incomplete, every `textureSample` returned (0,0,0,1), and `draw_glyph`'s
`color.a *= texel.a` painted the whole glyph quad.

`MIN_ARRAY_LAYERS = 2`, with the account at `create_array_texture` and a
`debug_assert!` there. Vulkan -- the phone's build and the desktop's
default backend -- was never affected.

`force-gles` now switches the desktop backend too, so the GLES path is
reproducible on a machine with a real GPU in seconds rather than only
through an APK: that is how this was found, with two shader probes
showing the sample was exactly (0,0,0,1).

docs/IRIS_TODO.md

@@ -609,17 +609,20 @@ agent ticks it here with the evidence.
   format rather than the palette -- but it makes the desktop build
   useless as a colour reference, which is exactly what P1a needed it for
   when the emulator could not draw glyphs.
-- [ ] **The emulator cannot draw iris's glyphs.** Under `-gpu host` with
-  Vulkan disabled (Mesa 26.2.2 / virgl -- what `emu` does on this
-  machine) every character renders as a solid filled box: the atlas
-  sample's alpha reads 1, which is what an incomplete GL texture returns
-  (0,0,0,1). Not new (`20303e0` does it too) and not the platform's
-  (Compose draws text perfectly on the same AVD in the same minute).
-  Enabling host Vulkan still dies at boot in gfxstream, and
-  `EMU_GPU=software` gives SwiftShader Vulkan on which iris **SIGSEGVs
-  in `surface_changed`**. Either of the last two would restore
-  appearance testing on Android; today it has to be done on the desktop
-  backend or on Iris's phone.
+- [x] **Every glyph was a solid box on the GLES backend -- iris's bug,
+  not the emulator's.** Fixed 2026-09-06. The atlas is one
+  `texture_2d_array` and `GpuTextures::new` created it with **one
+  layer**; wgpu-hal picks the GL target from the descriptor
+  (`(false, 1) => TEXTURE_2D`), so under GLES that array was a
+  `GL_TEXTURE_2D` bound to the shader's `sampler2DArray`, the unit was
+  incomplete, every `textureSample` returned (0,0,0,1), and
+  `draw_glyph`'s `color.a *= texel.a` filled the quad. `MIN_ARRAY_LAYERS
+  = 2` in `iris/core/src/render/texture.rs`, with a `debug_assert!` at
+  `create_array_texture`. Vulkan (the phone, the desktop's default
+  backend) was never affected. Reproduce the class in seconds without an
+  emulator: `iris`'s `force-gles` feature now switches the **desktop**
+  backend too -- `./run-headless.sh transcript --shot /tmp/x.png -- -p
+  transcript-ui --features iris/force-gles`.
 
 ## Build (for the port)
 

docs/RUST.md

@@ -5600,6 +5600,61 @@ device.
             transcript-ui -p desktop-app -p tabs-ui --all-targets`
             warning-free; `cargo test` 85 (iris, +4) + 13 (iris-core) +
             31 (transcript-ui, +11) + 123 (client-core).
+      **2026-09-06, after P1a: "the emulator cannot draw iris's glyphs"
+      was iris's bug, not the emulator's.** The finding recorded in the
+      box above -- that every glyph is a solid filled box under `-gpu
+      host` GLES and that this is what an incomplete GL texture returns
+      -- had the mechanism right and the attribution wrong. It is a real
+      defect on **any** adapter that is GL rather than Vulkan.
+      - **Reproduced off the emulator entirely**, which is what made it
+        cheap: `default/render.rs` now honours the same `force-gles`
+        feature `android/render.rs` did, so
+        `./run-headless.sh transcript --shot /tmp/x.png -- -p
+        transcript-ui --features iris/force-gles` draws the boxes on this
+        machine's own GPU in seconds. Two shader probes then said what
+        the sample was: `return vec4(texel.rgb, 1.0)` drew black boxes and
+        `return vec4(texel.a, texel.a, texel.a, 1.0)` drew white ones, so
+        the atlas sample was exactly (0, 0, 0, 1) -- GL's answer for an
+        **incomplete texture unit**, and not the null texture (which is
+        zeroed, alpha 0).
+      - **Root cause: the glyph atlas array was created with one layer.**
+        `GpuTextures::new` started `array_capacity` at 1 and `grow_array`
+        only doubles once a page needs a layer past it, so the ordinary
+        case -- one atlas page -- is a one-layer array. wgpu-hal picks the
+        GL texture target from the descriptor alone
+        (`gles::Texture::get_info_from_desc`: `(false, 1) => TEXTURE_2D`),
+        so that array is created as a `GL_TEXTURE_2D` and then bound to
+        the shader's `sampler2DArray`. wgpu has a name for this
+        (`log_failing_target_heuristics`, its issues #1614/#1574); the
+        result is an incomplete unit, `texel.a == 1`, and `draw_glyph`'s
+        `color.a *= texel.a` paints the whole glyph quad.
+      - **Fix**: `MIN_ARRAY_LAYERS = 2` in
+        `iris/core/src/render/texture.rs` -- the array is never created
+        with fewer, with the account at `create_array_texture` and a
+        `debug_assert!` there so a future capacity arithmetic change fails
+        at the mistake rather than as boxes on a screen. Cost: one page of
+        texture memory, which the next atlas page uses anyway.
+      - **Not a regression from `3e72a4e..20303e0`, and the bisect was not
+        run.** The defect is a function of the layer count, not of any
+        commit in that range: it has been there since the atlas became a
+        `texture_2d_array` (TEXTURES.md, 2026-09-04) and it reproduces at
+        HEAD and disappears at HEAD with the one-line capacity change. The
+        claimed "visible text at `3e72a4e`" is a misreading of its own
+        evidence -- `/tmp/final-typing.png`, the screenshot that entry
+        cites, is boxes; what the agent read was the `iris text render:
+        chars=5 glyphs=5` log line, which reports what **parley shaped**,
+        not what reached the screen. The earlier genuinely-good emulator
+        shots (`/tmp/after3.png`, 2026-09-05 19:56) predate the APK being
+        built with `force-gles` at all, so they were the Vulkan path.
+      - **The phone build is not affected and does not need withdrawing.**
+        `android-app/build-apk.sh`'s default features are deliberately
+        without `force-gles` (`d73db97`'s comment), so a phone build takes
+        `Backends::PRIMARY` -> Vulkan, where a one-layer array is an
+        ordinary one-layer array and glyphs draw correctly -- which is
+        also what Iris's phone reports have shown all along. The fix
+        matters for any device that falls back to GLES, which is why it is
+        not just an emulator convenience.
+
       - [ ] **P1b — tool-call cards and grouping.** `ToolRows.kt`/
             `ToolInput.kt`'s cards: a collapsed row per call with name
             and a one-line summary, expand to input and output, runs of

iris/Cargo.toml

@@ -66,7 +66,11 @@ send_wrapper = "0.6.0"
 # default) or virgl's GLES path, without a second env-var plumbing path that
 # nothing on this machine can hand to an already-launched Android process
 # (there is no `am start` environment and no system-property reader here to
-# add one). Android-only; `android/render.rs` is the only reader.
+# add one). Read by `android/render.rs` and, so the GLES path can be
+# reproduced on a machine with a real GPU rather than only in the emulator,
+# by `default/render.rs`:
+#     ./run-headless.sh transcript --shot /tmp/x.png -- -p transcript-ui \
+#         --features iris/force-gles
 force-gles = []
 
 [dev-dependencies]

iris/core/src/render/texture.rs

@@ -5,6 +5,10 @@ use crate::{PatchRect, TextureKind, TextureUpdate, Textures};
 
 use super::atlas::PAGE;
 
+/// The fewest layers the glyph atlas array is ever created with. Two, not
+/// one, for the GLES reason written on `create_array_texture`.
+const MIN_ARRAY_LAYERS: u32 = 2;
+
 /// What one texture slot is, GPU-side. Parallel to `Textures`' own slot
 /// numbering (`TextureKind`'s `Image`/`Page`), so a slot's index means the
 /// same thing on both sides without a second map to keep in sync.
@@ -360,7 +364,26 @@ impl GpuTextures {
         })
     }
 
+    /// The atlas is sampled as a `texture_2d_array`, and **a one-layer
+    /// array is not one on the GLES backend**: wgpu-hal picks the GL
+    /// texture target from the descriptor alone
+    /// (`gles::Texture::get_info_from_desc`, `(false, 1) => TEXTURE_2D`),
+    /// so a capacity of 1 creates a `GL_TEXTURE_2D` and binds it to the
+    /// shader's `sampler2DArray`. GL then treats that unit as incomplete
+    /// and every `textureSample` returns (0, 0, 0, 1) -- which, through
+    /// `draw_glyph`'s `color.a *= texel.a`, draws every glyph as a solid
+    /// filled box. That was iris's appearance on the emulator's GLES for
+    /// two days (RUST.md, "the emulator cannot draw iris's glyphs"), and
+    /// it is a real defect on any device whose adapter is GL rather than
+    /// Vulkan, not an emulator artifact. So the array never has fewer than
+    /// `MIN_ARRAY_LAYERS` layers; the second layer costs one page of
+    /// texture memory and is used by the next atlas page anyway.
     fn create_array_texture(device: &Device, capacity: u32) -> Texture {
+        debug_assert!(
+            capacity >= MIN_ARRAY_LAYERS,
+            "glyph atlas array asked for {capacity} layers; fewer than {MIN_ARRAY_LAYERS} is a \
+             GL_TEXTURE_2D on the GLES backend and draws every glyph as a box"
+        );
         device.create_texture(&TextureDescriptor {
             label: Some("glyph atlas array"),
             size: Extent3d {
@@ -382,7 +405,7 @@ impl GpuTextures {
     pub fn new(device: &Device, queue: &Queue) -> Self {
         let sampler = default_sampler(device);
         let null_view = null_texture_view(device);
-        let array_capacity = 1;
+        let array_capacity = MIN_ARRAY_LAYERS;
         let array_texture = Self::create_array_texture(device, array_capacity);
         let array_view = array_texture.create_view(&TextureViewDescriptor {
             dimension: Some(TextureViewDimension::D2Array),

iris/src/default/render.rs

@@ -85,7 +85,16 @@ impl UiRenderer {
         let size = window.inner_size();
 
         let instance = Instance::new(&InstanceDescriptor {
-            backends: Backends::PRIMARY,
+            // `force-gles` on the desktop too, not just on Android: the
+            // GLES backend has behaviour of its own (a one-layer array
+            // texture is a `GL_TEXTURE_2D` -- see
+            // `GpuTextures::create_array_texture`), and a machine with a
+            // real GPU is where that is cheap to reproduce and screenshot.
+            backends: if cfg!(feature = "force-gles") {
+                Backends::GL
+            } else {
+                Backends::PRIMARY
+            },
             ..Default::default()
         });