My daily job is to split the screen between figma and other apps and when I do so the working space its squeezed so you cant see anything and when you try to drag left or right bars to resize the space nothing happens.
Dont know where to post this so figma staff take action and do this feature since its important all other softwares has this.
Maybe not the desired solutions, but two tips anyway:
The left Sidebar is resizable when you place your cursor in the right spot. The rulers get in the way sometimes, so try to hide them and then grab the ride edge of the panel.
You can hide the UI with a right click on the canvas or cmd+\
