Xjyuk

I'm new to tikz and trying to create a simple diagram with some intersecting/overlapping rectangles. But I have some issues with aligning nodes within fit boxes, such that they overlap into the next boxes. I also need to clip/hide the background lines where they intersect. The figures should make it clear:

Desired output:

Current output:

Code:

documentclassarticle
usepackage[margin=20mm]geometry
usepackagetikz
usetikzlibrarychains,
 fit,
 positioning,
 
tikzstylelink = [->, thick, >=stealth]
makeatletter
tikzsetreset join/.code=deftikz@lib@on@chain
makeatother

begindocument
begintikzpicture[
node distance = 8mm and 6mm,
 start chain = going below,
box/.style = draw, thick, rounded corners,
 text width=10em, align=left, minimum height=1cm, minimum width=30mm,
 font=footnotesize,
cs2box/.style = draw, thick, rounded corners,
 text width=10em, align=left, minimum height=1cm, minimum width=100mm,
 font=footnotesize,
cs3box/.style = draw, thick, rounded corners,
 text width=10em, align=left, minimum height=1cm, minimum width=60mm,
 font=footnotesize,
FITout/.style = box, thin,inner xsep=1mm, inner ysep=6mm, yshift=5mm, fit=#1,
every label/.style = text width=11em, align=center, 
 font=footnotesizelinespread0.84selectfont
 ]
node (p1) [box] Case Study 1;
node (p2) [cs2box, below=1cm of p1] Case Study 2;
node (p3) [cs3box, below=1cm of p2] Case Study 3;
node (p4) [below=1cm of p3,box] Case Study 4;
node (p5) [below=1cm of p4,box] Case Study 5;
node[FITout=(p1) (p5), 
 label=[anchor=north]Phase 1] (x1) ;
node (p6) [right=1cm of x1, draw, thick, rounded corners,
text width=10em, align=left, minimum height=6cm, minimum width=30mm,
font=footnotesize, label=[anchor=north]Phase 2 ];
node (p7) [right=1cm of p6, draw, thick, rounded corners,
text width=10em, align=left, minimum height=6cm, minimum width=30mm,
font=footnotesize, label=[anchor=north]Phase 3 ];


endtikzpicture
enddocument

How do I align p2, p3 to start within the fit box x1? And clip the background lines when they intersect with the other nodes?

To align the nodes under each other on the left, I placed them under the west anchor and anchored each one to the west: below=1.5cm of p1.west,anchor=west

To hide the last placed nodes, I used the background library:

beginscope[on background layer]
node[FITout=(p1) (p5), 
 label=[anchor=north]Phase 1] (x1) ;
node (p6) [right=1cm of x1, draw, thick, rounded corners,
text width=10em, align=left, minimum height=6cm, minimum width=30mm,
font=footnotesize, label=[anchor=north]Phase 2 ];
node (p7) [right=1cm of p6, draw, thick, rounded corners,
text width=10em, align=left, minimum height=6cm, minimum width=30mm,
font=footnotesize, label=[anchor=north]Phase 3 ];
endscope

and I coloured the bottom of the node in white:

cs2box/.style = draw, thick, rounded corners,fill=white,
 text width=10em, align=left, minimum height=1cm, minimum width=120mm, font=footnotesize,
cs3box/.style = draw, thick, rounded corners,fill=white,
 text width=10em, align=left, minimum height=1cm, minimum width=60mm,
 font=footnotesize,

documentclassarticle
usepackage[margin=20mm]geometry
usepackagetikz
usetikzlibrarychains,
 fit,
 positioning,backgrounds
 
tikzstylelink = [->, thick, >=stealth]
makeatletter
tikzsetreset join/.code=deftikz@lib@on@chain
makeatother

begindocument
begintikzpicture[
node distance = 8mm and 6mm,
 start chain = going below,
box/.style = draw, thick, rounded corners,
 text width=10em, align=left, minimum height=1cm, minimum width=30mm, font=footnotesize,
cs2box/.style = draw, thick, rounded corners,fill=white,
 text width=10em, align=left, minimum height=1cm, minimum width=120mm, font=footnotesize,
cs3box/.style = draw, thick, rounded corners,fill=white,
 text width=10em, align=left, minimum height=1cm, minimum width=60mm,
 font=footnotesize,
FITout/.style = box, thin,inner xsep=1mm, inner ysep=6mm, yshift=5mm, fit=#1,
every label/.style = text width=11em, align=center, 
 font=footnotesizelinespread0.84selectfont
 ]
node (p1) [box] Case Study 1;
node (p2) [cs2box, below=1.5cm of p1.west,anchor=west] Case Study 2;
node (p3) [cs3box, below=1.5cm of p2.west,anchor=west] Case Study 3;
node (p4) [below=1.5cm of p3.west,anchor=west,box] Case Study 4;
node (p5) [below=1.5cm of p4.west,anchor=west,box] Case Study 5;
beginscope[on background layer]
node[FITout=(p1) (p5), 
 label=[anchor=north]Phase 1] (x1) ;
node (p6) [right=1cm of x1, draw, thick, rounded corners,
text width=10em, align=left, minimum height=6cm, minimum width=30mm,
font=footnotesize, label=[anchor=north]Phase 2 ];
node (p7) [right=1cm of p6, draw, thick, rounded corners,
text width=10em, align=left, minimum height=6cm, minimum width=30mm,
font=footnotesize, label=[anchor=north]Phase 3 ];
endscope

endtikzpicture
enddocument

Instead of a chain you could use a matrix, then it is less messy.

documentclassarticle
usepackage[margin=20mm]geometry
usepackagetikz
usetikzlibrarymatrix,fit,backgrounds
begindocument
begintikzpicture[font=sffamily]
matrix[matrix of nodes,nodes=anchor=west,minimum
height=1cm,align=center,draw,minimum width=30mm,fill=white,row sep=1em]
(mat) Case Study 3\
Case Study 4\
Case Study 5\
;
beginscope[on background layer,nodes=draw]
 node[fit=(mat-1-1)(mat-5-1),inner ysep=2em,yshift=1em,label=[anchor=north
 west]north west:Phase 1];
 node[fit=([xshift=5mm]mat-3-1.south east)([xshift=-20mm]mat-3-1.south
 east,inner ysep=2em,yshift=1em,label=[anchor=north
 west]north west:Phase 2];
 node[fit=(mat-2-1.south east)([xshift=-25mm]mat-2-1.south
 east,inner ysep=2em,yshift=1em,label=[anchor=north
 west]north west:Phase 3];
endscope
endtikzpicture
enddocument