LaTeX calculates the required space for floats incorrectly in some circumstances

[FrankMittelbach]

Brief outline of the bug

When LaTeX calculates the space requirements for floats it does so incorrectly if the float doesn't fit into the first area it tries. In the second and third area it adds additional space (from \textfloatsep or \floatsep) without removing \intextsep again. Thus, there are cases when a [t] fits but the same float as [ht] doesn't fit. Same, but less likely a problem: when [b] by its own fits [tb]or [hb] may not fit.

Minimal example showing the bug

\RequirePackage{latexbug}       % <--should be always the first line (see CONTRIBUTING)!
\documentclass{article}

 \setlength\intextsep{50pt}
\renewcommand\topfraction{.7}
\renewcommand\bottomfraction{.7}

\usepackage{fltrace}

\tracefloatvals

\tracefloats

\begin{document}

aaa\par aaa\par aaa\par aaa\par aaa\par aaa\par  
aaa\par aaa\par aaa\par aaa\par aaa\par aaa\par 
aaa\par aaa\par aaa\par aaa\par aaa\par aaa\par 
aaa\par aaa\par aaa\par aaa\par aaa\par aaa\par 

\begin{figure}[htb]
  1\rule{2cm}{8cm}
\end{figure}

bbb\par bbb\par bbb\par bbb\par bbb\par bbb\par 
bbb\par bbb\par bbb\par bbb\par bbb\par bbb\par 
bbb\par bbb\par bbb\par bbb\par bbb\par bbb\par 
bbb\par bbb\par bbb\par bbb\par bbb\par bbb\par 
bbb\par bbb\par bbb\par bbb\par bbb\par bbb\par 
\end{document}

Log file (required) and possibly PDF file

Relevant portion of the log \@reqcolroom should not change from one test to the next:

LaTeX2e: ***Start addtocurcol
LaTeX2e: (mod 32) fpstype: 23
LaTeX2e: (mult of 32) currtype: 32
LaTeX2e: ORD float
LaTeX2e: fpstype: 23(flsettextmin)
LaTeX2e: ORD textmin = 109.99832pt
LaTeX2e: textfloatsheight (before) = 0.0pt
LaTeX2e: textmin + textfloatsheight: 109.99832pt
LaTeX2e: page-so-far: 286.0pt
LaTeX2e: float size = 227.62204pt(addtocurcol)
LaTeX2e: colroom = 550.0pt(addtocurcol)
LaTeX2e: reqcolroom = 513.62204pt(addtocurcol)
LaTeX2e: fpstype: 23(flsetnum \@colnum)
LaTeX2e: \@colnum  (before) = 3
LaTeX2e: \@currbox single column float -- ok
LaTeX2e: deferlist: : (addtocurcol-before)
LaTeX2e: botlist: : (addtocurcol-before)
LaTeX2e: fpstype OK here: 23
LaTeX2e: Fail---no room at 2nd test of colroom (addtocorcol \intextsep)
LaTeX2e: not here: sent to addtotoporbot
LaTeX2e: ***Start addtotoporbot
LaTeX2e: fpstype OK top: 23
LaTeX2e: fpstype: 23(flsetnum \@topnum)
LaTeX2e: \@topnum  (before) = 2
LaTeX2e: colroom = 550.0pt(flcheckspace \@toproom \@toplist)
LaTeX2e: reqcolroom = 583.62204pt(flcheckspace \@toproom \@toplist)
LaTeX2e: Fail---no room at 2nd test of colroom (flcheckspace \@toproom \@toplis
t)
LaTeX2e: sent to addtobot (addtotoporbot)
LaTeX2e: ***Start addtobot
LaTeX2e: fpstype OK bot: 23
LaTeX2e: fpstype: 23(flsetnum \@botnum)
LaTeX2e: \@botnum  (before) = 1
LaTeX2e: colroom = 550.0pt(flcheckspace \@botroom \@botlist)
LaTeX2e: reqcolroom = 603.62204pt(flcheckspace \@botroom \@botlist)
LaTeX2e: Fail---no room at 2nd test of colroom (flcheckspace \@botroom \@botlis
t)
LaTeX2e: put on deferlist (addtocurcol)
LaTeX2e: deferlist: \bx@A : (addtocurcol-after)