The sidebar's width was a GtkPaned divider position: `window - 940` in pixels. On any window
narrower than 940 px the divider landed past the right edge and the sidebar came up 0 px wide,
reachable only by dragging the divider back.
Every attempt that kept the paned failed for that reason -- deriving the position from the paned's
own width, re-parenting the column, property-form children, code-set size requests. The mechanism was
the fault, so it is gone: the row is an AdwOverlaySplitView, which is what a utility sidebar is in
GNOME, with min-sidebar-width = max-sidebar-width = 320 in pixels. Its width is therefore its own at
any window size, and when there is no room beside the plot it *overlays* the plot (collapsed, set from
the window's width in tick()) instead of being squeezed to nothing.
Two things found on the way, both recorded in the invariants:
- sidebar-width-fraction is a 0..1 fraction of the window whatever sidebar-width-unit says. Setting a
pixel count there is a GObject critical and it falls back to 0.25, which is why the sidebar first
read 560. The width is pinned with min/max instead.
- the window opened at its content minimum (756 px, the plot column plus the sidebar) because the
explicit gtk_window_set_default_size() was dropped when the window moved into the .ui. Restored.
[sidebar-probe] reads both states and is the guard: window=756 sidebar=320 under a 2x scale, which is
the case that used to read 0.
Martino Ferrari
bb01278d88