Agent Browser Viewport

agent-browser viewport sizing

The default agent-browser viewport (1280x720) is too short for many real-world web apps, causing elements to overlap or scroll unexpectedly. This skill sizes the agent-browser Chromium window to fill its current screen, leaving a configurable bottom margin for the desktop panel / status bar.

Usage

.agents/skills/agent-browser-viewport/scripts/fit-viewport.py        # default 50px bottom margin
.agents/skills/agent-browser-viewport/scripts/fit-viewport.py 80     # larger panel

Takes effect immediately on the current agent-browser session — no reload needed. Run once before starting automation. Re-run if the window has been moved to a different physical monitor.

Why a script, not a fixed -N fudge

agent-browser set viewport <w> <h> sets the outer Chromium window size, but Chromium then adds its tab strip + URL bar on top, so the actual window ends up larger than the value you pass. The overhead varies by Chromium version, theme, devicePixelRatio and window manager — no fixed -120 constant is reliable across machines.

How the script works

It derives Chromium's chrome overhead directly from the existing window state (no probe / resize loop, so the page layout isn't disturbed mid-fit):

• horizontal overhead = outerWidth - innerWidth × devicePixelRatio
• vertical overhead = outerHeight - innerHeight × devicePixelRatio

innerWidth / innerHeight are CSS pixels; outerWidth / outerHeight are device pixels. Multiplying inner by DPR brings them into the same unit, and the difference is the chrome (tab strip + URL bar + window border) in device pixels — exactly what set viewport needs as its correction.

The script then:

1. Reads the screen the agent-browser window is currently on via window-screen-info -n .agent-browser-data (device pixels). Works correctly on multi-monitor setups.
2. Reads the in-page innerWidth / innerHeight / outerWidth / outerHeight / devicePixelRatio via agent-browser eval.
3. Computes overhead as above.
4. Issues one agent-browser set viewport (screen_width - over_w, screen_height - bottom_margin - over_h) call.

The final outer window matches screen_width × (screen_height - bottom_margin) exactly (modulo half-pixel rounding at non-integer DPR).

Notes

• The script identifies the agent-browser window by its WM_CLASS instance .agent-browser-data — Chromium sets this from the --user-data-dir path that agent-browser passes.
• Reported screen dimensions are device pixels. agent-browser set viewport also takes device pixels, so values pass through directly. At devicePixelRatio > 1 the in-page innerWidth / innerHeight will be smaller (CSS pixels = device pixels ÷ DPR), which is expected.
• If you need a different bottom margin (e.g. taller panel, multi-row taskbar), pass it as the first argument.