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I've often wanted to set the size of an interface element to something like Full - 50, so as to not over-constrain box sizes.

One approach to this would be to simply use a Dynamic[boxsize - 50] as my ImageSize.

Unfortunately, for boxes that information isn't possible to extract by normal means.

I've always resorted to hacks and workarounds, but I don't see what that info shouldn't be possible to extract from the FE.

In digging I found FrontEnd`GetBoundingBoxSizePacket, but it always returns one of two things. Either it gives:

FrontEndExecute[FrontEnd`GetBoundingBoxSizePacket@EvaluationCell[]] {{34., 13., 7.}}

Or 

FrontEndExecute[FrontEnd`GetBoundingBoxSizePacket@1] $Failed

No matter what FE object I pass to it, it only ever returns one size.

Does anyone know can I get the size of a BoxObject?

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1 Answer 1

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GetBoundingBoxSizePacket seems to only operate on Cell expressions, so we can do:

getBoxSize[c_Cell] := {#[[1]], Total@#[[2 ;;]]} &@ First@FrontEndExecute@GetBoundingBoxSizePacket[c]; getBoxSize[c_CellObject] := getBoxSize[NotebookRead[c]]; getBoxSize[b_BoxObject] := getBoxSize[ Cell[BoxData@NotebookRead[b], "Output", PageWidth -> Infinity, ShowCellBracket -> False, CellMargins -> {{0, 0}, {0, 0}} ] ]; getBoxSize[e_] := getBoxSize[ Cell[BoxData@ToBoxes[e], "Output", PageWidth -> Infinity, ShowCellBracket -> False, CellMargins -> {{0, 0}, {0, 0}} ] ];

And we get what we expect:

c = EvaluationCell[]; getBoxSize@EvaluationCell[] Rasterize[c, "RasterSize"] {215., 60.} {215, 60} Dynamic[boxo = EvaluationBox[]] Rasterize[Dynamic[EvaluationBox[]], "RasterSize"] getBoxSize@boxo {136, 26} {135., 26.}
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