main.content (.content-builder) inherited "margin:0 auto" from the
base .content class. On this page body is a column flex container, so
that auto margin disabled cross-axis stretch and made the element
shrink-to-fit instead — sized via an algorithm that runs back down
through the flex/%/aspect-ratio chain to the canvas, and shifts
slightly whenever sibling panel content changes (e.g. selecting an
element). Give it an explicit width:100% so max-width/auto-margin
centering still work but sizing is stable and fills the available
space. Also add min-width:0 to .builderPanelRight, matching its
siblings, as defense against the same class of bug.
Verified with Playwright: canvas box was previously not filling
available width (918px expected vs 379px actual at a 1920px viewport)
and shifted between steps; both are fixed.
william