HTML Web page sectioned into grids with draggable panes
This is an Apps Script Web App.
Most of this app is HTML, CSS and JavaScript.
The Web App is an example of CSS grids.
The CSS has 5 grid columns.
Two of the grid columns are for drag bars that are only a few pixels wide.
The app has 3 panes in the main body which can be resized using left and right drag bars.
The way that the resizing of the grid sections is done also allows the sections to be resized when the browser size is changed.
CSS grid has a special size unit named "fr" which is short for "fraction" but is a "flex" capability.
So CSS grid has the same capability of a flex-box but there are a lot more options with CSS grid.