bugs - How can I change the thickness of tick marks in BarLegend?

Bug introduced in 10.1 or earlier and persisting through 11.0.1 or later

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This answer explains how to change various undocumented options for BarLegend with Method. In particular, I want to change the style of the ticks (marks and labels) in BarLegend. For example,

BarLegend[{"SunsetColors", {0, 1}}, LabelStyle -> {FontSize -> 12},

 Method -> {Frame -> False, TicksStyle -> Directive[Red, AbsoluteThickness[2]]}]

However, in Mathematica 10.2 on Linux, BarLegend refuses to change the thickness of the tick marks. Also, if I do not add the LabelStyle option then the labels don't change color? In Plot this command works,

Plot[Sin[x], {x, 0, 2 Pi},TicksStyle -> Directive[Red, AbsoluteThickness[2]]]

so I would expect that it works for BarLegend as well.

What is going on and is there a workaround to change the thickness of the tick marks? Thanks.

Answer

Somehow the AbsolutThickness you specified gets replaced by a default value of AbsoluteThickness[0.2].

This misbehavior can be corrected by replacing the incorrect value with your specification.

PlotLegends; (*preload definitions*)

Cell[BoxData[
   MakeBoxes@
      BarLegend[{"SunsetColors", {0, 1}}, LabelStyle -> {FontSize -> 12}, 
       Method -> {Frame -> False, TicksStyle -> Directive[Red, AbsoluteThickness[2]]}] /. 
     Directive[RGBColor[1, 0, 0], AbsoluteThickness[_]] -> 
      Directive[RGBColor[1, 0, 0], AbsoluteThickness[2]] // #[[1, 1]] &
   ], "Output"] // CellPrint

For opaque ticks:

Cell[BoxData[
   MakeBoxes@
      BarLegend[{"SunsetColors", {0, 1}}, LabelStyle -> {FontSize -> 12}, 
       Method -> {Frame -> False, TicksStyle -> Directive[Red, AbsoluteThickness[2]]}] /. 
     Directive[RGBColor[1, 0, 0], AbsoluteThickness[_]] -> 
      Directive[RGBColor[1, 0, 0], AbsoluteThickness[2], Opacity[1]] // #[[1, 1]] &
   ], "Output"] // CellPrint

---

Correcting the BarLegend of a DensityPlot, using the syntax provided in the answer by Praan :

DensityPlot[Sin[x y], {x, 0, 1}, {y, 0, 1}, 
  PlotLegends -> 
   BarLegend[Automatic, LabelStyle -> {FontSize -> 12}, 
    Method -> {Frame -> False, TicksStyle -> Directive[Red, AbsoluteThickness[2]]}]] /. 
 Placed[barLegend_BarLegend, args__] :> 
  Placed[ToExpression[
    FrameBox @@ MakeBoxes[barLegend] /. 

      Directive[Red, AbsoluteThickness[_]] -> 
       Directive[Red, AbsoluteThickness[2], Opacity[1]]], args]

The same output can be achieved by using the following LegendFunction

DensityPlot[Sin[x y], {x, 0, 1}, {y, 0, 1}, 
 PlotLegends -> 
  BarLegend[Automatic, LabelStyle -> {FontSize -> 12}, 
   Method -> {Frame -> False, 
     TicksStyle -> Directive[Red, AbsoluteThickness[2]]}, 

   LegendFunction -> (# /. 
       Directive[Red, AbsoluteThickness[_]] -> 
        Directive[Red, AbsoluteThickness[2], Opacity[1]] &)]]

---

With the answer by Praan and our discussion in the comments it became clear, that a wrong InterpretationFunction inside the TemplateBox created by BarLegend can cause additional problems.
Compare

MakeBoxes[
    BarLegend[{"SunsetColors", {0, 1}}, LegendMarkerSize -> 300, 
     LabelStyle -> {FontSize -> 12}, 

     Method -> {FrameStyle -> Black, AxesStyle -> None, 
       TicksStyle -> Black}]] /. 
   AbsoluteThickness[_] -> 
    AbsoluteThickness[2] /. (InterpretationFunction :> 
     f_) -> (InterpretationFunction :> (# &)) // ToExpression

with

MakeBoxes[
   BarLegend[{"SunsetColors", {0, 1}}, LegendMarkerSize -> 300, 

    LabelStyle -> {FontSize -> 12}, 
    Method -> {FrameStyle -> Black, AxesStyle -> None, 
      TicksStyle -> Black}]] /. 
  AbsoluteThickness[_] -> AbsoluteThickness[2] // ToExpression

or just the InterpretationFunction

MakeBoxes[
    BarLegend[{"SunsetColors", {0, 1}}, LegendMarkerSize -> 300, 
     LabelStyle -> {FontSize -> 12}, 

     Method -> {FrameStyle -> Black, AxesStyle -> None, 
       TicksStyle -> Directive[Black, AbsoluteThickness[2]]}]] /. 
   AbsoluteThickness[_] -> 
    AbsoluteThickness[2] // #[[-1, 2, 1]] & // ToExpression

and the first code block in the answer by Praan.