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Here is a strange behavior reminding me of a previous bug when Manipulate updates without reason, but it is probably not the same reason here and seems to be specific to ParametricPlot. (My specs: OSX, MMA 11.2)

First evaluate this command:

Manipulate[
 ParametricPlot[r*{Cos[t], Sin[t]}, {t, 0, 2 Pi}, 
  PlotRange -> {-2, 2}, PlotTheme -> None, 
  PlotLabel -> RandomReal[]], {r, 1, 2}]

then observe that this previous cell updates when you evaluate in a separate cell:

t=1

If you set t to another value, Manipulate will evaluates again.

What causes that behavior and is there a workaround ?

(Note that I set PlotTheme->None just in case that option would be responsible for that. Also there is no problem with Plotfor example so it seems to be function specific.) )

enter image description here

Update

It becomes even more strange as other Manipulate begin to update the same way only when the Manipulate + ParametricPlot is present:

enter image description here

You can check that if you delete the output of Manipulate[ParametricPlot[...], the Manipulate[Table[... won't update anymore if you change the value of t (actually it will update only once then no more ...).

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    $\begingroup$ It looks like a flaw with how Dynamic decides which symbols are supposed to be tracked. Or which not to include, it can handle e.g. Module[{t}, or Plot (I see it is triggered once) but fails for ParametricPlot, PolarPlot. Just an observation. $\endgroup$
    – Kuba
    Commented Mar 9, 2018 at 12:02
  • $\begingroup$ @Kuba I added an another observation: it seems the tracking problem is not local. It explains also your observation i see it is triggered once ... $\endgroup$
    – SquareOne
    Commented Mar 9, 2018 at 14:00
  • $\begingroup$ I'd report this, btw Dynamic[ParametricPlot[t,{t,0,1}]] is enough to show it so that we don't have to think about all fancy stuff behind Manipulate. $\endgroup$
    – Kuba
    Commented Mar 9, 2018 at 19:53

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