Generalized the floating-panel mechanism from the properties panel (props/propsPanelFloat/propsPanelReopenBtn) into reusable functions keyed by a short id, and applied it to the left "Éléments de cet écran" panel (key "left", panelLeftFloat, panelLeftReopenBtn). Both panels can now be closed independently, letting the canvas take the screen's full width for the most faithful possible preview against the actual /play rendering. Shared CSS (.floatPanel/.floatPanelHeader/.floatPanelBody/.floatPanelReopenBtn with --left/--right position variants) replaces the props-panel-only rules from the previous commit.
Fixed the two querySelector('.builderPanel .elementTree') call sites (used to patch the tree preview into the DOM after an unrelated canvas-only refresh) that would have silently stopped finding the tree once its wrapper's class changed.
william
850242d1d4