fix: Size the expanded dialog and grid columns to their content (#2611)

- Closes #2607

docs/configuration/grid-layout-algorithm.md

@@ -27,3 +27,18 @@ grid_max_columns]` that will fit at least a `600px` column width.
 - Otherwise, use the largest number of columns in the range `[2 -
 grid_max_columns]` that will fit at least a `190px` column width.
 - Otherwise, there will be `1` column only.
+
+Unless `grid_columns` is specified, that number is then reduced to the number of
+columns the items actually need. Surplus columns would otherwise be left empty
+and every item shrunk to a width it did not need -- a single camera taking half
+the grid, for example.
+
+The number of columns needed is the sum of each item's
+[`width_factor`](cameras/README.md?id=dimensions), rounded up to a whole number
+of columns per item, plus enough extra columns for any one of them to be
+selected (see `grid_selected_width_factor` in [`live`](live.md?id=display) or
+[`media_viewer`](media-viewer.md?id=display)). That room is reserved whether or
+not there is a selection, so selecting an item never changes the number of
+columns -- which would otherwise resize the items the user did not interact
+with. A grid with a single item is an exception, as that item can never be wider
+than the whole grid.