Panel:SizeToChildren
Description
Resizes the panel to fit the bounds of its children.
Your panel must have its layout updated (Panel:InvalidateLayout) for this function to work properly.
The sizeW and sizeH parameters are false by default. Therefore, calling this function with no arguments will result in a no-op.
Arguments
Example
Using Panel:InvalidateLayout
local Frame = vgui.Create( "DFrame" )
Frame:SetSize( 300, 400 )
Frame:Center()
Frame:MakePopup()
Frame:SetSizable( true )
-- with :InvalidateLayout(true)
local backgroundPanel = vgui.Create( "DPanel", Frame )
backgroundPanel:Dock( TOP )
backgroundPanel:DockPadding( 4, 4, 4, 4 )
backgroundPanel:DockMargin( 0, 0, 0, 4 )
local button1 = vgui.Create( "DButton", backgroundPanel )
button1:Dock( TOP )
button1:DockMargin( 0, 0, 0, 4 )
button1:SetTall( 60 )
button1:SetText( "c1" )
local button2 = vgui.Create( "DButton", backgroundPanel )
button2:Dock( TOP )
button2:SetTall( 60 )
button2:SetText( "c2" )
backgroundPanel:InvalidateLayout( true )
backgroundPanel:SizeToChildren( false, true )
-- now w/o :InvalidateLayout
local backgroundPanel = vgui.Create( "DPanel", Frame )
backgroundPanel:Dock( TOP )
backgroundPanel:DockPadding( 4, 4, 4, 4 )
local button1 = vgui.Create( "DButton", backgroundPanel )
button1:Dock( TOP )
button1:DockMargin( 0, 0, 0, 4 )
button1:SetTall( 60 )
button1:SetText( "c1" )
local button2 = vgui.Create( "DButton", backgroundPanel )
button2:Dock( TOP )
button2:SetTall( 60 )
button2:SetText( "c2" )
backgroundPanel:SizeToChildren( false, true )
Output: 
Garry's Mod
Rust
Steamworks
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S&box