Remove white dialog background in MSI user interface.

We received a report that if you enable Windows 10's high-contrast
mode, the text in PuTTY's installer UI becomes invisible, because it's
displayed in the system default foreground colour against a background
of the white right-hand side of our 'msidialog.bmp' image. That's fine
when the system default fg is black, but high-contrast mode flips it
to white, and now you have white on white text, oops.

Some research in the WiX bug tracker suggests that in Windows 10 you
don't actually have to use BMP files for your installer images any
more: you can use PNG, and PNGs can be transparent. However, someone
else reported that that only works in up-to-date versions of Windows.

And in fact there's no need to go that far. A more elegant answer is
to simply not cover the whole dialog box with our background image in
the first place. I've reduced the size of the background image so that
it _only_ contains the pretty picture on the left-hand side, and omits
the big white rectangle that used to sit under the text. So now the
RHS of the dialog is not covered by any image at all, which has the
same effect as it being covered with a transparent image, except that
it doesn't require transparency support from msiexec. Either way, the
background for the text ends up being the system's default dialog-box
background, in the absence of any images or controls placed on top of
it - so when the high-contrast mode is enabled, it flips to black at
the same time as the text flips to white, and everything works as it
should.

The slight snag is that the pre-cooked WiX UI dialog specifications
let you override the background image itself, but not the Width and
Height fields in the control specifications that refer to them. So if
you just try to drop in a narrow image in the most obvious way, it
gets stretched across the whole window.

But that's not a show-stopper, because we're not 100% dependent on
getting WiX to produce exactly the right output. We already have the
technology to postprocess the MSI _after_ it comes out of WiX: we're
using it to fiddle the target-platform field for the Windows on Arm
installers. So all I had to do was to turn msiplatform.py into a more
general msifixup.py, add a second option to change the width of the
dialog background image, and run it on the x86 installers as well as
the Arm ones.

Buildscr

@@ -152,8 +152,7 @@ in putty do perl -i~ -pe 'y/\015//d;s/$$/\015/' LICENCE
 
 # Some gratuitous theming for the MSI installer UI.
 in putty/icons do make -j$(nproc)
-in putty do convert -size 164x312 'gradient:blue-white' -distort SRT -90 -swirl 180 \( -size 329x312 canvas:white \) +append \( icons/putty-48.png -geometry +28+24 \) -composite \( icons/pscp-48.png -geometry +88+96 \) -composite \( icons/puttygen-48.png -geometry +28+168 \) -composite \( icons/pageant-48.png -geometry +88+240 \) -composite windows/msidialog.bmp
-in putty do convert -size 493x58 canvas:white \( icons/putty-48.png -geometry +440+5 \) -composite windows/msibanner.bmp
+in putty do ./windows/make_install_images.sh
 
 mkdir putty/windows/build32
 mkdir putty/windows/build64
@@ -200,10 +199,20 @@ in putty/windows with wixonlinux do candle -arch x64 -dRealPlatform=x64 -dDllOk=
 in putty/windows with wixonlinux do candle -arch x64 -dRealPlatform=Arm -dDllOk=no -dBuilddir=abuild32/ -dWinver="$(Winver)" -dPuttytextver="$(Puttytextver)" installer.wxs && light -ext WixUIExtension -ext WixUtilExtension -sval installer.wixobj -o installera32.msi -spdb
 in putty/windows with wixonlinux do candle -arch x64 -dRealPlatform=Arm64 -dDllOk=no -dBuilddir=abuild64/ -dWinver="$(Winver)" -dPuttytextver="$(Puttytextver)" installer.wxs && light -ext WixUIExtension -ext WixUtilExtension -sval installer.wixobj -o installera64.msi -spdb
 
-# Bodge the platform fields for the Windows on Arm installers, since
-# WiX 3 doesn't understand Arm platform names itself.
-in putty/windows do ./msiplatform.py installera32.msi Arm
-in putty/windows do ./msiplatform.py installera64.msi Arm64
+# Change the width field for our dialog background image so that it
+# doesn't stretch across the whole dialog. (WiX's default one does; we
+# replace it with a narrow one so that the text to the right of it
+# shows up on system default background colour, meaning that
+# high-contrast mode doesn't make the text white on white. But that
+# means we also have to modify the width field, and there's nothing in
+# WiX's source syntax to make that happen.)
+#
+# Also bodge the platform fields for the Windows on Arm installers,
+# since WiX 3 doesn't understand Arm platform names itself.
+in putty/windows do ./msifixup.py installer32.msi --dialog-bmp-width=123
+in putty/windows do ./msifixup.py installer64.msi --dialog-bmp-width=123
+in putty/windows do ./msifixup.py installera32.msi --dialog-bmp-width=123 --platform=Arm
+in putty/windows do ./msifixup.py installera64.msi --dialog-bmp-width=123 --platform=Arm64
 
 # Sign the Windows installers.
 ifneq "$(cross_winsigncode)" "" in putty/windows do $(cross_winsigncode) -i https://www.chiark.greenend.org.uk/~sgtatham/putty/ -n "PuTTY Installer" installer32.msi installer64.msi installera32.msi installera64.msi

windows/make_install_images.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+# Script to make the bitmap files that go into the PuTTY MSI installer.
+
+set -e
+
+# For convenience, allow this script to be run from the Windows
+# subdirectory as well as the top level of the source tree.
+if test -f installer.wxs -a ! -f putty.h -a -f ../putty.h; then
+  cd ..
+fi
+
+convert -size 164x312 'gradient:blue-white' -distort SRT -90 -swirl 180 \
+        \( icons/putty-48.png -geometry +28+24 \) -composite \
+        \( icons/pscp-48.png -geometry +88+96 \) -composite \
+        \( icons/puttygen-48.png -geometry +28+168 \) -composite \
+        \( icons/pageant-48.png -geometry +88+240 \) -composite \
+        windows/msidialog.bmp
+
+convert -size 493x58 canvas:white \
+        \( icons/putty-48.png -geometry +440+5 \) -composite \
+        windows/msibanner.bmp

windows/msifixup.py

@@ -16,30 +16,43 @@ def run(command, verbose):
         sys.stdout.write("".join(
             "> {}\n".format(line) for line in out.splitlines()))
 
-def set_platform(msi, platform, verbose):
-    run(["msidump", "-t", msi], verbose)
+def make_changes(msi, args):
+    run(["msidump", "-t", msi], args.verbose)
+    build_cmd = ["msibuild", msi]
 
-    summary_stream = "_SummaryInformation.idt"
+    def change_table(filename):
+        with open(filename) as fh:
+            lines = [line.rstrip("\r\n").split("\t")
+                     for line in iter(fh.readline, "")]
 
-    with open(summary_stream) as fh:
-        lines = [line.rstrip("\r\n").split("\t")
-                 for line in iter(fh.readline, "")]
+        for line in lines[3:]:
+            yield line
 
-    for line in lines[3:]:
-        if line[0] == "7":
-            line[1] = ";".join([platform] + line[1].split(";", 1)[1:])
+        with open(filename, "w") as fh:
+            for line in lines:
+                fh.write("\t".join(line) + "\r\n")
 
-    with open(summary_stream, "w") as fh:
-        for line in lines:
-            fh.write("\t".join(line) + "\r\n")
+        build_cmd.extend(["-i", filename])
 
-    run(["msibuild", msi, "-i", summary_stream], verbose)
+    if args.platform is not None:
+        for line in change_table("_SummaryInformation.idt"):
+            if line[0] == "7":
+                line[1] = ";".join([args.platform] + line[1].split(";", 1)[1:])
+
+    if args.dialog_bmp_width is not None:
+        for line in change_table("Control.idt"):
+            if line[9] == "WixUI_Bmp_Dialog":
+                line[5] = args.dialog_bmp_width
+
+    run(build_cmd, args.verbose)
 
 def main():
     parser = argparse.ArgumentParser(
         description='Change the platform field of an MSI installer package.')
     parser.add_argument("msi", help="MSI installer file.")
-    parser.add_argument("platform", help="New value for the platform field.")
+    parser.add_argument("--platform", help="Change the platform field.")
+    parser.add_argument("--dialog-bmp-width", help="Change the width field"
+                        " in all uses of WixUI_Bmp_Dialog.")
     parser.add_argument("-v", "--verbose", action="store_true",
                         help="Log what this script is doing.")
     parser.add_argument("-k", "--keep", action="store_true",
@@ -51,7 +64,7 @@ def main():
     try:
         tempdir = tempfile.mkdtemp(dir=msidir)
         os.chdir(tempdir)
-        set_platform(msi, args.platform, args.verbose)
+        make_changes(msi, args)
     finally:
         if args.keep:
             sys.stdout.write(