Try this. Press F1 to activate:

F1::
    ToggleBorderlessWindow()
Return

; https://github.com/ahkon/Fullscreen-window-toggle
ToggleBorderlessWindow(hwnd:="") {
    static MONITOR_DEFAULTTONEAREST := 0x00000002
    static WS_CAPTION               := 0x00C00000
    static WS_SIZEBOX               := 0x00040000
    static WindowStyle              := WS_CAPTION|WS_SIZEBOX
    static A                        := []
    if (!hwnd)
        hwnd := WinExist("A")
    Win := "ahk_id " hwnd                                                          ; Store WinTitle
    WinGet, S, Style, % Win                                                      ; Get window style
    if (S & WindowStyle) {                                                      ; If not borderless
        A[Win, "Style"] := S & WindowStyle                                   ; Store existing style
        WinGet, IsMaxed, MinMax, % Win                  ; Get/store whether the window is maximized
        if (A[Win, "Maxed"] := IsMaxed = 1 ? true : false)
            WinRestore, % Win
        WinGetPos, X, Y, W, H, % Win                                   ; Store window size/location
        A[Win, "X"] := X, A[Win, "Y"] := Y, A[Win, "W"] := W, A[Win, "H"] := H
        WinSet, Style, % -WindowStyle, % Win                                       ; Remove borders
        hMon := DllCall("User32\MonitorFromWindow", "Ptr", hwnd, "UInt", MONITOR_DEFAULTTONEAREST)
        VarSetCapacity(monInfo, 40), NumPut(40, monInfo, 0, "UInt")
        DllCall("User32\GetMonitorInfo", "Ptr", hMon, "Ptr", &monInfo)
        WinMove, % Win,,  monLeft   := NumGet(monInfo,  4, "Int")          ; Move and resize window
                    ,  monTop    := NumGet(monInfo,  8, "Int")
                    , (monRight  := NumGet(monInfo, 12, "Int")) - monLeft
                    , (monBottom := NumGet(monInfo, 16, "Int")) - monTop
    }
    else if A[Win] {                                                                ; If borderless
        WinSet, Style, % "+" A[Win].Style, % Win                                  ; Reapply borders
        WinMove, % Win,, A[Win].X, A[Win].Y, A[Win].W, A[Win].H       ; Return to original position
        if (A[Win].Maxed)                                                    ; Maximize if required
            WinMaximize, % Win
        A.Delete(Win)
    }
}