Browser zoom capability repro

Date: 2026-08-31 Target: http://127.0.0.1:34401 (rendered login page, /auth/login) Browser: Google Chrome for Testing 152.0.7977.64 CLI: agent-browser 0.27.0

Result

True browser zoom is repeatable through a disposable Manifest V3 extension loaded by agent-browser. The extension calls chrome.tabs.setZoom(tabId, factor) with 0.8, 1.25, or 2.0.

This satisfies the requested invariant at every factor and viewport tested:

visualViewport.scale stays 1.
CSS viewport dimensions change by the inverse zoom factor.
devicePixelRatio tracks the browser zoom factor.
The rendered h1 bounds change with the layout.

The agent-browser press Control+- path is not native browser zoom. It dispatches a page key event and left all measured values unchanged. The CLI reference also has no direct Chrome browser-zoom command. Do not relabel Emulation.setPageScaleFactor, CSS zoom, or viewport/device emulation as browser zoom; those are separate mechanisms.

The provided vault login helper was attempted without reading vault metadata or credential values. The rendered login page was sufficient for this browser-level probe, so no product interaction or mutation was required.

Repeatable method

Create a disposable extension with tabs permission and host permission limited to the candidate app. Set ZOOM_FACTOR to the desired value.

manifest.json:

{
  "manifest_version": 3,
  "name": "Disposable native zoom probe",
  "version": "1.0.0",
  "permissions": ["tabs"],
  "host_permissions": ["http://127.0.0.1:34401/*"],
  "background": { "service_worker": "background.js" }
}

background.js:

const ZOOM_FACTOR = 1.25; // replace with 0.8, 1.25, or 2.0

function applyZoom(tab) {
  if (tab?.id == null || !tab.url?.startsWith("http://127.0.0.1:34401")) return;
  chrome.tabs.setZoom(tab.id, ZOOM_FACTOR);
}

chrome.tabs.onUpdated.addListener((tabId, changeInfo, tab) => {
  if (changeInfo.status === "complete") applyZoom({ ...tab, id: tabId });
});

chrome.tabs.query({}, (tabs) => tabs.forEach(applyZoom));

Start a fresh, uniquely named session and disposable profile. On this headless host, extensions require explicit --headless=new; keep --no-sandbox in the launch args. The non-extension baseline sessions used the exact --args '--no-sandbox' form:

npx -y --package agent-browser agent-browser \
  --session zoom-extension-<factor>-<unique> \
  --profile /tmp/agent-browser-zoom-profile-<factor>-<unique> \
  --extension /tmp/agent-browser-native-zoom-extension-<factor>-<unique> \
  --headed false \
  --args '--no-sandbox,--headless=new' \
  open http://127.0.0.1:34401

Set the exact viewport and measure the same fixed element:

npx -y --package agent-browser agent-browser \
  --session zoom-extension-<factor>-<unique> set viewport 320 568

npx -y --package agent-browser agent-browser \
  --session zoom-extension-<factor>-<unique> eval \
  "(()=>{const e=document.querySelector('h1')||document.body;const r=e.getBoundingClientRect();const v=window.visualViewport;return {innerWidth,innerHeight,outerWidth,outerHeight,devicePixelRatio,visualViewport:{width:v?.width,height:v?.height,scale:v?.scale},fixed:{x:r.x,y:r.y,width:r.width,height:r.height}}})()"

Capture the screenshot, then close the session. Use a fresh profile or clear the disposable profile before testing another factor:

npx -y --package agent-browser agent-browser \
  --session zoom-extension-<factor>-<unique> screenshot \
  /absolute/path/.screenshots/release-v2/repro-browser-zoom-capability/native-<factor>-320x568.png
npx -y --package agent-browser agent-browser \
  --session zoom-extension-<factor>-<unique> close

The same method was run at 1920 1080. A native extension zoom session must be restarted when changing the factor; changing only the viewport is safe.

Measurements

The fixed probe is the first rendered h1 (Log in to your account). Bounds are CSS pixels in x, y, width, height order. outer is the browser window metric reported by Chromium; it is intentionally included because it stays at the window size while the CSS viewport changes.

320x568 requested viewport

Zoom	inner (W x H)	outer (W x H)	devicePixelRatio	visualViewport (W x H / scale)	fixed h1 bounds
100% baseline	320 x 568	500 x 711	1	305 x 568 / 1	87, 177, 191, 57.59375
80% native	400 x 710	500 x 711	0.800000011920929	381.25 x 710 / 1	87.2265625, 201.23046875, 266.7578125, 28.7890625
125% native	256 x 454	500 x 711	1.25	244 x 454.3999938964844 / 1	86.80000305175781, 176.8000030517578, 130.40000915527344, 86.4000015258789
200% native	160 x 284	500 x 711	2	152.5 x 284 / 1	87, 177, 95.5703125, 86.390625

1920x1080 requested viewport

Zoom	inner (W x H)	outer (W x H)	devicePixelRatio	visualViewport (W x H / scale)	fixed h1 bounds
100% baseline	1920 x 1080	1920 x 1223	1	1920 x 1080 / 1	765, 418.71875, 340.71875, 28.796875
80% native	2400 x 1350	1920 x 1223	0.800000011920929	2400 x 1350 / 1	1005.21484375, 562.734375, 340.64453125, 28.7890625
125% native	1536 x 864	1920 x 1223	1.25	1536 x 864 / 1	572.7999877929688, 320.20001220703125, 340.7250061035156, 28.80000114440918
200% native	960 x 540	1920 x 1223	2	952.5 x 540 / 1	281.25, 295.3984375, 340.71875, 28.796875

Negative controls

At 320x568, press Control+- produced no change from the 100% baseline: innerWidth=320, innerHeight=568, outerWidth=500, outerHeight=711, devicePixelRatio=1, visualViewport=305x568, scale=1, and the fixed h1 remained 87,177,191,57.59375. The corresponding capture is keyboard-control-noop-320x568.png.

Pre-seeding a disposable profile with profile.default_zoom_level, partition.default_zoom_level, and a localhost per_host_zoom_levels entry also did not alter metrics when launched through agent-browser; it is not a reliable path here. A first extension launch without --headless=new exited before CDP with a missing X server error.

Evidence artifacts

All captures are 8-bit PNGs at the requested pixel dimensions under .screenshots/release-v2/repro-browser-zoom-capability/:

baseline-320x568-100.png
baseline-1920x1080-100.png
native-80-320x568.png, native-80-1920x1080.png
native-125-320x568.png, native-125-1920x1080.png
native-200-320x568.png, native-200-1920x1080.png
keyboard-control-noop-320x568.png

No product tests, builds, Docker compilation, or E2E tests were run.

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