diff --git a/src/GlyphRenderer.ts b/src/GlyphRenderer.ts
index 742f0879ff4f04509e4a07c8efdf0d5743fe8ee5..885a206a394fff783737a412c0b75bf935dd0eca 100644
--- a/src/GlyphRenderer.ts
+++ b/src/GlyphRenderer.ts
@@ -61,6 +61,8 @@ function createFragmentShaderSource(maxFragmentShaderTextureUnits: number): stri
   for (let i = 1; i < maxFragmentShaderTextureUnits; i++) {
     textureConditionals += ` else if (v_texpage == ${i}) { outColor = texture(u_texture[${i}], v_texcoord); }`;
   }
+  // A v_texpage beyond the sampler budget matches no branch above. Leaving outColor unwritten
+  // is undefined behaviour in GLSL ES and paints garbage, so fall through to transparent.
   return (`#version 300 es
 precision lowp float;
 
@@ -74,7 +76,7 @@ out vec4 outColor;
 void main() {
   if (v_texpage == 0) {
     outColor = texture(u_texture[0], v_texcoord);
-  } ${textureConditionals}
+  } ${textureConditionals} else { outColor = vec4(0.0, 0.0, 0.0, 0.0); }
 }`);
 }
 
diff --git a/src/TextureAtlas.ts b/src/TextureAtlas.ts
index eff0d0e1c36906b1f14b85fb8db7907bf685c8a7..b4fcff8c6be49b1868a17bcabfa3bb54689fb8b1 100644
--- a/src/TextureAtlas.ts
+++ b/src/TextureAtlas.ts
@@ -3,6 +3,7 @@
  * @license MIT
  */
 
+import { FontWeight } from '@xterm/xterm';
 import { IColorContrastCache } from 'browser/Types';
 import { DIM_OPACITY, TEXT_BASELINE } from './Constants';
 import { tryDrawCustomGlyph } from './customGlyphs/CustomGlyphRasterizer';
@@ -114,21 +115,26 @@ export class TextureAtlas implements ITextureAtlas {
     this._onRemoveTextureAtlasCanvas.dispose();
   }
 
-  public warmUp(): void {
+  public warmUp(domContainer?: HTMLElement): void {
     if (!this._didWarmUp) {
-      this._doWarmUp();
+      this._doWarmUp(domContainer);
       this._didWarmUp = true;
     }
   }
 
-  private _doWarmUp(): void {
+  private _doWarmUp(domContainer: HTMLElement | undefined): void {
     // Pre-fill with ASCII 33-126, this is not urgent and done in idle callbacks
     this._warmUpQueue?.clear();
     const queue = this._warmUpQueue = new IdleTaskQueue(this._logService);
     for (let i = 33; i < 126; i++) {
       queue.enqueue(() => {
+        // Orca: a canvas outside the document ignores the page's font smoothing and rasterizes
+        // heavier glyphs that stay cached, so leave them to on-demand drawing until connected.
+        if (!domContainer?.isConnected) {
+          return;
+        }
         if (!this._cacheMap.get(i, DEFAULT_COLOR, DEFAULT_COLOR, DEFAULT_EXT)) {
-          const rasterizedGlyph = this._drawToCache(i, DEFAULT_COLOR, DEFAULT_COLOR, DEFAULT_EXT, false, undefined);
+          const rasterizedGlyph = this._drawToCache(i, DEFAULT_COLOR, DEFAULT_COLOR, DEFAULT_EXT, false, domContainer);
           this._cacheMap.set(i, DEFAULT_COLOR, DEFAULT_COLOR, DEFAULT_EXT, rasterizedGlyph);
         }
       });
@@ -138,21 +144,23 @@ export class TextureAtlas implements ITextureAtlas {
   private _pageLayoutVersion = 0;
   public get pageLayoutVersion(): number { return this._pageLayoutVersion; }
 
+  // Orca diagnostics: read by terminal-render-desync-weight-probe.ts to tell a real
+  // bold-collapse from a repaint problem. Canvas silently keeps its previous font when an
+  // assignment fails to parse, which rasterizes glyphs at a stale weight.
+  public fontProbeMismatchCount = 0;
+  public fontProbeLastMismatch: { desired: string, actual: string } | undefined;
+
   public clearTexture(): void {
-    if (this._pages[0].currentRow.x === 0 && this._pages[0].currentRow.y === 0) {
+    // Guard on every page rather than pages[0]: a merged page is never written through
+    // currentRow, so once one lands at index 0 the old check made every later clear a no-op.
+    if (this._pages.every(page => page.glyphs.length === 0 && page.currentRow.x === 0 && page.currentRow.y === 0)) {
       return;
     }
-    for (const page of this._pages) {
-      page.clear();
-    }
-    this._cacheMap.clear();
-    this._cacheMapCombined.clear();
-    this._didWarmUp = false;
-
-    // Invalidate renderer models so all texture pages are refreshed. The atlas may be shared, in
-    // which case the clearing renderer has cleared only its own model and every other owner still
-    // holds texture coords into the rows just wiped.
-    this._pageLayoutVersion++;
+    // Return the atlas to its constructor state instead of clearing in place: page.clear() leaves
+    // page.glyphs populated, which would keep the guard above from ever firing again. Eviction
+    // also bumps _pageLayoutVersion, so every renderer sharing this atlas rebuilds its model.
+    this._evictAllPages();
+    this._createNewPage();
   }
 
   private _createNewPage(): AtlasPage {
@@ -468,6 +476,36 @@ export class TextureAtlas implements ITextureAtlas {
     return this._config.colors.contrastCache;
   }
 
+  /**
+   * Orca diagnostic. Canvas ignores a font assignment it cannot parse and silently keeps the
+   * previous value, so a bad family or weight rasterizes every glyph at a stale weight. Record
+   * the mismatch rather than correcting it: the goal is to tell that failure apart from a
+   * repaint bug when a terminal renders bold-collapsed.
+   */
+  private _probeRasterizationFontWeight(fontWeight: FontWeight): void {
+    const desired = String(fontWeight);
+    // Only numeric weights are comparable; keywords round-trip through Canvas unchanged.
+    if (!/^(?:[1-8]\d{2}|900)$/.test(desired)) {
+      return;
+    }
+    // Canvas normalizes font serialization: Chromium omits 400 and emits the keyword bold for 700.
+    const token = this._tmpCtx.font.match(
+      /(?:^|\s)(normal|bold|[1-9]\d{0,3})(?=\s+\d+(?:\.\d+)?px(?:\s|$))/
+    )?.[1] ?? '400';
+    const actual = token === 'normal' ? '400' : token === 'bold' ? '700' : token;
+    if (actual === desired) {
+      return;
+    }
+    this.fontProbeMismatchCount++;
+    this.fontProbeLastMismatch = { desired, actual: this._tmpCtx.font };
+    try {
+      (globalThis as { __orcaAtlasFontProbe?: (mismatch: { desired: string, actual: string }) => void })
+        .__orcaAtlasFontProbe?.(this.fontProbeLastMismatch);
+    } catch {
+      // Diagnostics only; a throwing listener must never break rasterization.
+    }
+  }
+
   private _drawToCache(codeOrChars: number | string, bg: number, fg: number, ext: number, restrictToCellHeight: boolean, domContainer: HTMLElement | undefined): IRasterizedGlyph {
     const chars = typeof codeOrChars === 'number' ? String.fromCharCode(codeOrChars) : codeOrChars;
 
@@ -539,6 +577,7 @@ export class TextureAtlas implements ITextureAtlas {
     const fontStyle = italic ? 'italic' : '';
     this._tmpCtx.font =
       `${fontStyle} ${fontWeight} ${this._config.fontSize * this._config.devicePixelRatio}px ${this._config.fontFamily}`;
+    this._probeRasterizationFontWeight(fontWeight);
     this._tmpCtx.textBaseline = TEXT_BASELINE;
 
     const powerlineGlyph = chars.length === 1 && isPowerlineGlyph(chars.charCodeAt(0));
diff --git a/src/Types.ts b/src/Types.ts
index bfd6fd76cebe8f55dce7bfad5406d4f7cd184320..0bd283ea7b2bb72d5d3f20b6e7e11e7cc44f283f 100644
--- a/src/Types.ts
+++ b/src/Types.ts
@@ -65,7 +65,7 @@ export interface ITextureAtlas extends IDisposable {
   /**
    * Warm up the texture atlas, adding common glyphs to avoid slowing early frame.
    */
-  warmUp(): void;
+  warmUp(domContainer?: HTMLElement): void;
 
   /**
    * Incremented whenever cached glyph texture page mappings may be stale, such as after atlas page
diff --git a/src/WebglRenderer.ts b/src/WebglRenderer.ts
index 2d0f5a3a8ee7fc0765a0179cee0102edb9376e4b..92f1c7241e887cd907d21d91aa26c0def02339f5 100644
--- a/src/WebglRenderer.ts
+++ b/src/WebglRenderer.ts
@@ -323,7 +323,7 @@ export class WebglRenderer extends Disposable implements IRenderer {
       );
     }
     this._charAtlas = atlas;
-    this._charAtlas.warmUp();
+    this._charAtlas.warmUp(this._terminal.element);
     this._glyphRenderer.value?.setAtlas(this._charAtlas);
   }
 
@@ -393,7 +393,10 @@ export class WebglRenderer extends Disposable implements IRenderer {
     // page's version, so re-run the update and force a full texture rebind.
     let merged = false;
     let mergeRetries = 0;
-    while (this._charAtlas && this._glyphRenderer.value.beginFrame() && mergeRetries++ < Constants.MERGE_RETRY_LIMIT) {
+    // Test the retry budget before beginFrame: beginFrame latches the page layout version it
+    // observed, so tripping the limit after consuming it would strand a stale model with no
+    // later frame able to notice it needs rebuilding.
+    while (this._charAtlas && mergeRetries++ < Constants.MERGE_RETRY_LIMIT && this._glyphRenderer.value.beginFrame()) {
       merged = true;
       this._clearModel(true);
       this._updateModel(0, this._terminal.rows - 1);
@@ -403,6 +406,20 @@ export class WebglRenderer extends Disposable implements IRenderer {
     }
 
     // Render
+    // A transparent framebuffer keeps the previous draw, so re-blending translucent
+    // backgrounds accumulated opacity on every redraw and flickered (Orca #6491).
+    if (this._terminal.options.allowTransparency) {
+      // Keep the RGB blend but write source alpha as-is; SRC_ALPHA on alpha
+      // itself would turn the configured opacity `a` into `a²`.
+      this._gl.blendFuncSeparate(
+        this._gl.SRC_ALPHA,
+        this._gl.ONE_MINUS_SRC_ALPHA,
+        this._gl.ONE,
+        this._gl.ONE_MINUS_SRC_ALPHA
+      );
+      this._gl.clearColor(0, 0, 0, 0);
+      this._gl.clear(this._gl.COLOR_BUFFER_BIT);
+    }
     this._rectangleRenderer.value.renderBackgrounds();
     this._glyphRenderer.value.render(this._model);
     if (!this._cursorBlinkStateManager.value || this._cursorBlinkStateManager.value.isCursorVisible) {
diff --git a/src/renderLayer/BaseRenderLayer.ts b/src/renderLayer/BaseRenderLayer.ts
index 5b11615328957b6f5d41a174284c11e061910d91..1654e5ae39baf65a7243cbe7b1395c28824b269e 100644
--- a/src/renderLayer/BaseRenderLayer.ts
+++ b/src/renderLayer/BaseRenderLayer.ts
@@ -97,7 +97,7 @@ export abstract class BaseRenderLayer extends Disposable implements IRenderLayer
     }
 
     this._charAtlas = acquireTextureAtlas(terminal, this._optionsService.rawOptions, colorSet, this._deviceCellWidth, this._deviceCellHeight, this._deviceCharWidth, this._deviceCharHeight, this._coreBrowserService.dpr, 2048);
-    this._charAtlas.warmUp();
+    this._charAtlas.warmUp(terminal.element);
   }
 
   public resize(terminal: Terminal, dim: IRenderDimensions): void {