Fix sprocket preview canvas switching

[lexluthor0304]

Summary

• keep the 2D canvas visible when sprocket preview is toggled on in Step 3
• restore the WebGL preview display size from the current image source when sprocket preview is toggled off
• prevent the non-sprocket preview from inheriting the sprocket frame aspect ratio

Verification

• node negative2positive/src/app/sprocketFrame.test.mjs
• node negative2positive/src/app/imageDataOps.test.mjs
• npm run build:web
• Chrome DevTools: SAMPLE.jpg -> Positive Mode -> Step 3 -> Sprocket Preview on/off restores original preview aspect ratio

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Project	Deployment	Actions	Updated (UTC)
negative-converter	Ready	Preview, Comment	Jun 10, 2026 12:02pm

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reset the backing canvas when returning to WebGL

When sprocket preview is turned off in Step 3, updatePreview() now takes the WebGL path, and this line only restores the CSS display size. The hidden 2D canvas dimensions remain set to the sprocket-framed size from renderFastSprocketPreview/setMainCanvasDimensions, so any later code that uses canvas.width/canvas.height still sees the framed aspect; for example the window resize handler calls adjustCanvasDisplay(canvas.width, canvas.height), which re-applies the sprocket aspect after a resize. Reset the backing canvas dimensions to the non-sprocket source size when switching back to WebGL.

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