Retained tabs stay mounted under display:none while inactive, which
reports a zero-width onLayout. ControlledAgentControls fed that
straight into its density resolver, so a hidden tab's toolbar
collapsed to its tightest density and painted one frame collapsed
before re-expanding when the tab became visible again.
Extract the zero-width guard that useContainerWidthBelow already had
into a shared readMeasuredWidth helper, and use it in the composer
toolbar's own layout handler so a display:none measurement can never
drive its density.
Also bump the chat outline rail's MIN_PANEL_WIDTH breakpoint (818 ->
918) so the rail disappears earlier under a narrower panel, and widen
the chat-outline e2e spec's viewport so its panel keeps healthy margin
over the new threshold.
Measure the header row width and hide text labels on the Open,
Scripts, and git action split buttons below 700px so the header
fits without wrapping on split-pane layouts.
