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Renderer resizes and fully repaints every frame at a fractional devicePixelRatio (and leaves seams between cell backgrounds)

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
72/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
typescript
Domain
frontend

Research direction

Start in the CanvasRenderer.render() and resize() paths mentioned in the report, then reproduce the issue at a fractional devicePixelRatio such as 1.36. Verify the canvas dimensions are compared using the same integer sizing that resize() applies, and check that idle frames no longer resize or fully repaint and that coloured cell backgrounds have no visible seams.

Written by the indexing model from the issue text.

Description

Version: ghostty-web 0.4.0-next.20.g1858a59 (also current main, 1858a59)

What happens
With a non-integer window.devicePixelRatio (browser zoom other than 100%, a scaled display, e.g.
1.36 or 2.2), the canvas renderer calls resize() and does a full redraw on every animation
frame, about 60 times a second while nothing changes. The caret and output look "busy", and CPU
use goes up. Cell backgrounds are also drawn at fractional device-pixel edges, which leaves
hairline seams (a visible grid) over coloured backgrounds.

Cause
CanvasRenderer.render() checks

this.canvas.width !== D.cols * this.metrics.width * this.devicePixelRatio

canvas.width is always an integer and the right-hand side is fractional at a fractional DPR, so
the check is true on every frame and resize() runs again, forcing a full redraw.

Repro
Open the demo, set browser zoom to 110% (or run Chrome with --force-device-scale-factor=1.36),
then wrap term.renderer.resize to count calls: about 60/s while idle. At DPR 1 or 2: 0/s.

Fix suggestion
Compare against Math.round(cols * width * dpr) (as resize() sets it), or render at
Math.ceil(devicePixelRatio) and let the browser scale the canvas down. The second also removes
the seams. We work around it by setting renderer.devicePixelRatio = Math.ceil(window.devicePixelRatio)
after open(), re-applied on a resolution media-query change. That took idle resizes from 60/s
to 0 and removed the seams.

Dominant language
TypeScript
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