Index: src/diff.c
==================================================================
--- src/diff.c
+++ src/diff.c
@@ -351,10 +351,12 @@
       appendDiffLine(pOut, ' ', &B[b+j], html);
     }
   }
 }
 
+static int maxwidth;
+
 /*
 ** Status of a single output line
 */
 typedef struct SbsLine SbsLine;
 struct SbsLine {
@@ -388,10 +390,14 @@
   int k;   /* Cursor position */
   int needEndSpan = 0;
   const char *zIn = pLine->z;
   char *z = &p->zLine[p->n];
   int w = p->width;
+
+  if (n > maxwidth)
+    maxwidth = n;
+
   for(i=j=k=0; k<w && i<n; i++, k++){
     char c = zIn[i];
     if( p->escHtml ){
       if( i==p->iStart ){
         int x = strlen(p->zStart);
@@ -863,11 +869,11 @@
 static void sbsDiff(
   DContext *p,       /* The computed diff */
   Blob *pOut,        /* Write the results here */
   int nContext,      /* Number of lines of context around each change */
   int width,         /* Width of each column of output */
-  int escHtml        /* True to generate HTML output */
+  int escHtml       /* True to generate HTML output */
 ){
   DLine *A;     /* Left side of the diff */
   DLine *B;     /* Right side of the diff */  
   int a = 0;    /* Index of next line in A[] */
   int b = 0;    /* Index of next line in B[] */
@@ -1432,11 +1438,10 @@
 ** Extract the width of columns for side-by-side diff.  Supply an
 ** appropriate default if no width is given.
 */
 int diff_width(int diffFlags){
   int w = (diffFlags & DIFF_WIDTH_MASK)/(DIFF_CONTEXT_MASK+1);
-  if( w==0 ) w = 80;
   return w;
 }
 
 /*
 ** Generate a report of the differences between files pA and pB.
@@ -1492,10 +1497,18 @@
   if( pOut ){
     /* Compute a context or side-by-side diff into pOut */
     int escHtml = (diffFlags & DIFF_HTML)!=0;
     if( diffFlags & DIFF_SIDEBYSIDE ){
       int width = diff_width(diffFlags);
+      if (width == 0){ /* Autocalculate */
+        Blob dump;
+        /* Webserver */
+        maxwidth = 0;
+        blob_zero(&dump);
+        sbsDiff(&c, &dump, nContext, width, escHtml);
+        width = maxwidth;
+      }
       sbsDiff(&c, pOut, nContext, width, escHtml);
     }else{
       int showLn = (diffFlags & DIFF_LINENO)!=0;
       contextDiff(&c, pOut, nContext, showLn, escHtml);
     }

Index: src/diffcmd.c
==================================================================
--- src/diffcmd.c
+++ src/diffcmd.c
@@ -41,12 +41,19 @@
     /* no-op */
   }else if( diffFlags & DIFF_SIDEBYSIDE ){
     int w = diff_width(diffFlags);
     int n1 = strlen(zLeft);
     int x;
-    if( n1>w*2 ) n1 = w*2;
-    x = w*2+17 - (n1+2);
+    if (w > 0) {
+        if( n1>w*2 ) n1 = w*2;
+        x = w*2+17 - (n1+2);
+    }else{
+        /* Autocalculate width
+         * We can't know the width in advance, so we'll make it
+         * output three = around the name */
+        x = 6;
+    }
     z = mprintf("%.*c %.*s %.*c\n",
                 x/2, '=', n1, zLeft, (x+1)/2, '=');
   }else{
     z = mprintf("--- %s\n+++ %s\n", zLeft, zRight);
   }

Index: src/info.c
==================================================================
--- src/info.c
+++ src/info.c
@@ -363,12 +363,13 @@
   }else{
     int x;
     if( sideBySide ){
       diffFlags = DIFF_SIDEBYSIDE | DIFF_IGNORE_EOLWS;
 
-      /* "dw" query parameter determines width of each column */
-      x = atoi(PD("dw","80"))*(DIFF_CONTEXT_MASK+1);
+      /* "dw" query parameter determines width of each column
+       * 0 means autocalculate. */
+      x = atoi(PD("dw","0"))*(DIFF_CONTEXT_MASK+1);
       if( x<0 || x>DIFF_WIDTH_MASK ) x = DIFF_WIDTH_MASK;
       diffFlags += x;
     }else{
       diffFlags = DIFF_INLINE | DIFF_IGNORE_EOLWS;
     }