Highlight the Bounding Box, Part I: Cartesian Grid
VBA Excel, 150 bytes 146 bytes

Instruction:

Create a workbook with two blank worksheets: Sheet1 and Sheet2. Set the input in Sheet1 and then put the following code in the Sheet1 code module

Sub A:For Each C In UsedRange:If C.Value="#"Then Sheet2.Range(C.Address)="#"
Next:For Each C In Sheet2.UsedRange:Range(C.Address)="#":Next:End Sub


Ungolfed the code:

Sub A()

For Each C In UsedRange
    If C.Value = "#" Then Sheet2.Range(C.Address) = "#"
Next

For Each C In Sheet2.UsedRange
    Range(C.Address) = "#"
Next

End Sub


Explanation:


Loop through every cell in the used range Sheet1
Set the conditional statement to copy every cell contains character hashtag (#) in the used range Sheet1 and paste it to the cell in Sheet2 with the same address as Sheet1.
Loop through once again every cell in the used range Sheet2 to copy every cell address in it and then use it to assign character hashtag (#) to the cell in Sheet1 with the same address as the used range Sheet2.


Example I/O:

  



Caveat: Make sure every cell in Sheet2 always blank every time you run the program.