From 38f0091c64523cbda8c080c0525c74e725f40c87 Mon Sep 17 00:00:00 2001
From: Adam Malczewski <github@tradam.dev>
Date: Mon, 15 Jun 2026 15:16:57 +0900
Subject: rml-compositing: make the window field actually render (2 real bugs)
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Diagnosed live (nested unbox + grim screenshots). Two bugs stopped any window
from showing; both fixed and visually confirmed (multi-window, live-updating,
focus highlight, RCSS tiling).

1. kernel (ui_substrate.cpp, adopt_node) — STABLE TEXTURE ID across re-imports.
   The live import did glDeleteTextures+glGenTextures every commit, minting a NEW
   GL id each frame. But RmlUi's image()/<img> decorator caches the texture HANDLE
   it gets from LoadTexture(uri) ONCE, so after the first re-import it drew a
   DELETED texture -> blank. Now node.tex is allocated ONCE and just re-pointed at
   the new EGLImage (dmabuf) / re-uploaded (shm) on each re-import, destroying the
   previous EGLImage after the rebind. The id stays constant => RmlUi's cached
   handle stays valid => the window updates live. (Reimport counting unchanged.)

2. ext-window-field assets — the window texture is painted via an RCSS image()
   DECORATOR bound with data-style-decorator (the stage-dock pattern), NOT an
   <img src="{{ w.live_uri }}">: RmlUi does not substitute a data binding inside
   an img src, so the literal "{{ w.live_uri }}" was reaching the texture loader
   ("Could not load texture"). Also `contain` (was `cover`) so the whole window
   shows instead of cropping its center.

Verified: foot composites as a surface element in the window field and updates
live (a ticking clock); a 2nd window tiles with the focused one highlighted.
kernel + ext-window-field suites green; build-asan kernel green (no leak/UB).

Follow-up: a kernel regression test asserting the texture id is stable across
re-imports (no test caught this — RmlUi caching is the subtlety).
---
 assets/ext-window-field/field.rcss   |  8 +++++---
 assets/ext-window-field/field.rml    | 16 ++++++++++------
 packages/kernel/src/ui_substrate.cpp | 33 ++++++++++++++++++++-------------
 3 files changed, 35 insertions(+), 22 deletions(-)

diff --git a/assets/ext-window-field/field.rcss b/assets/ext-window-field/field.rcss
index da84358..5e7d5e6 100644
--- a/assets/ext-window-field/field.rcss
+++ b/assets/ext-window-field/field.rcss
@@ -65,9 +65,11 @@ div.win.focused {
     transform: scale(1.0);
 }
 
-/* The live window texture fills its tile 1:1 (the <img src=live_uri> of the
-   surface element). The tile's rounded overflow:hidden clips it to the card. */
-div.win img.live {
+/* The live window texture fills its tile (the surface element's live_uri, painted
+   via an RCSS `image()` decorator bound in field.rml — the proven dynamic-texture
+   pattern, since RmlUi does NOT substitute data bindings in an <img src>). The
+   tile's rounded overflow:hidden clips it to the card. */
+div.win .live {
     display: block;
     position: absolute;
     left: 0;
diff --git a/assets/ext-window-field/field.rml b/assets/ext-window-field/field.rml
index 15f031e..7fc2d95 100644
--- a/assets/ext-window-field/field.rml
+++ b/assets/ext-window-field/field.rml
@@ -8,14 +8,18 @@
      flag. LAYOUT + ANIMATION are RCSS (field.rcss) — the user's contract
      decision: C++ never computes a window's on-screen rectangle.
 
-     Each row renders one <img src="{{ w.live_uri }}"> = the window's live
-     texture (UiSubstrate::create_surface_element -> SurfaceElement::source_uri()).
-     The kernel manages the toplevel's whole subsurface/popup tree behind that
-     single <img>; the document addresses only the root. data-class-focused keys
-     the RCSS highlight/raise off the focused window. -->
+     Each row paints the window's LIVE texture via an RCSS `image()` decorator
+     bound with data-style-decorator (the proven dynamic-texture pattern — RmlUi
+     does NOT substitute a data binding inside an <img src>, so the URI must reach
+     RCSS, not the src attribute). live_uri = SurfaceElement::source_uri() from
+     UiSubstrate::create_surface_element. `contain` shows the whole window
+     (letterboxed) rather than cropping. The kernel manages the toplevel's whole
+     subsurface/popup tree behind that single element; the document addresses only
+     the root. data-class-focused keys the RCSS highlight/raise off the focused
+     window. -->
 <body data-model="ui" class="field">
 <div data-for="w : wins" class="win" data-class-focused="w.focused">
-<img class="live" src="{{ w.live_uri }}"/>
+<div class="live" data-style-decorator="'image( ' + w.live_uri + ' contain center center )'"/>
 </div>
 </body>
 </rml>
diff --git a/packages/kernel/src/ui_substrate.cpp b/packages/kernel/src/ui_substrate.cpp
index fd42a44..5971c56 100644
--- a/packages/kernel/src/ui_substrate.cpp
+++ b/packages/kernel/src/ui_substrate.cpp
@@ -1613,15 +1613,18 @@ bool Substrate::Impl::adopt_node(SurfaceElementState& el, SurfaceNode& node) {
         EGLImageKHR img =
             gl.egl_create_image(gl.egl_display, EGL_NO_CONTEXT, EGL_LINUX_DMA_BUF_EXT, nullptr, ia);
         if (img != EGL_NO_IMAGE_KHR) {
-            // Drop the old GL objects, build the new texture from the EGLImage.
-            if (node.tex != 0) {
-                glDeleteTextures(1, &node.tex);
-                node.tex = 0;
+            // STABLE TEXTURE ID across re-imports. RmlUi's image()/<img> decorator
+            // caches the texture HANDLE it gets from LoadTexture(uri) ONCE (at load
+            // time) and draws that handle every frame — so if we regenerated the GL
+            // id on each re-import, a live window would show its FIRST frame then go
+            // blank (it would draw a DELETED texture). Allocate node.tex ONCE and
+            // just re-point it at the NEW EGLImage on every re-import, destroying the
+            // PREVIOUS image after the rebind. The id stays constant => the cached
+            // RmlUi handle stays valid => the window updates live.
+            EGLImageKHR old_image = node.image;
+            if (node.tex == 0) {
+                glGenTextures(1, &node.tex);
             }
-            if (node.image != EGL_NO_IMAGE_KHR && gl.egl_destroy_image != nullptr) {
-                gl.egl_destroy_image(gl.egl_display, node.image);
-            }
-            glGenTextures(1, &node.tex);
             glBindTexture(GL_TEXTURE_2D, node.tex);
             gl.gl_image_target_texture(GL_TEXTURE_2D, static_cast<GLeglImageOES>(img));
             glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
@@ -1629,6 +1632,9 @@ bool Substrate::Impl::adopt_node(SurfaceElementState& el, SurfaceNode& node) {
             glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
             glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
             glBindTexture(GL_TEXTURE_2D, 0);
+            if (old_image != EGL_NO_IMAGE_KHR && gl.egl_destroy_image != nullptr) {
+                gl.egl_destroy_image(gl.egl_display, old_image);
+            }
             node.image = img;
             node.width = attribs.width;
             node.height = attribs.height;
@@ -1647,15 +1653,16 @@ bool Substrate::Impl::adopt_node(SurfaceElementState& el, SurfaceNode& node) {
         wlr_buffer_unlock(buf); // import failed: drop the lock we just took
         return node.tex != 0;
     }
-    if (node.tex != 0) {
-        glDeleteTextures(1, &node.tex);
-        node.tex = 0;
-    }
+    // STABLE TEXTURE ID (see the dmabuf path above): reuse node.tex so RmlUi's
+    // cached decorator handle stays valid; a re-upload (glTexImage2D) into the
+    // same id refreshes the pixels for an shm client's live updates.
     if (node.image != EGL_NO_IMAGE_KHR && gl.egl_destroy_image != nullptr) {
         gl.egl_destroy_image(gl.egl_display, node.image);
         node.image = EGL_NO_IMAGE_KHR;
     }
-    glGenTextures(1, &node.tex);
+    if (node.tex == 0) {
+        glGenTextures(1, &node.tex);
+    }
     glBindTexture(GL_TEXTURE_2D, node.tex);
     glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_SWIZZLE_R, GL_BLUE);
     glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_SWIZZLE_B, GL_RED);
-- 
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