Strange response with dynamic texture and MousePosition

Question

I built a cubic, then use MousePosition together with GUIScreenShot to take a floating region with fixed size (200*200) on the screen, and use it as dynamic texture on the cubic. My code is

Needs["GUIKit`"];
$HistoryLength = 1;

DynamicModule[{mpp},
 Dynamic@{
  Refresh[
   ClearSystemCache[];
   g = GUIScreenShot[With[{mp = MousePosition[]}, mpp = {mp - 100, mp + 100}]];
   Graphics3D[{Texture[g], Polygon[coords, VertexTextureCoordinates -> Table[vtc, {6}]]}, Lighting -> "Neutral"],
   UpdateInterval -> .5],
  mpp, g},
 Initialization :> (
  vtc = {{0, 0}, {0, 1}, {1, 1}, {1, 0}};
  coords = {{{0, 0, 0}, {0, 1, 0}, {1, 1, 0}, {1, 0, 0}}, {{0, 0, 0}, {1, 0, 0}, {1, 0, 1}, {0, 0, 1}}, {{1, 0, 0}, {1, 1, 0}, {1, 1, 1}, {1, 0, 1}}, {{1, 1, 0}, {0, 1, 0}, {0, 1, 1}, {1, 1, 1}}, {{0, 1, 0}, {0, 0, 0}, {0, 0, 1}, {0, 1, 1}}, {{0, 0, 1}, {1, 0, 1}, {1, 1, 1}, {0, 1, 1}}};)]

You can see the result by executing the code then moving your mouse around. Like this,

But there are two strange problems,

1) the size of the floating area taken should be always 200*200, however it will actually change.

2) the memory usage will constantly grow as we move our mouse, but I have already used $HistoryLength=1 and ClearSystemCache[].

Any suggestion?

Answer 1

Look very carefully at the documentation. This had me puzzled for a while too:

When the argument consists of screen coordinates of the form {{xmin,xmax},{ymin,ymax}}, a rectangle from the current screen is captured.

The key part is {min,max} like PlotRange. Your code produces {{min,min},{max,max}}.

I think the memory usage problem is a memory leak in the front end Java code. I don't think there's anything you can do about it except report it to WRI. If you really want to investigate, VisualVM might be helpful, though I have never tried it with Mathematica.