Avoid negative space in \bmod. (#984)

This is a workaround for #836 for the command case of using \bmod,
without changing any spacing (and making the code and generated output
slightly more efficient).  In particular, fix #982.

src/functions/mod.js

@@ -12,13 +12,16 @@ const htmlModBuilder = (group, options) => {
     const inner = [];
 
     if (group.value.modType === "bmod") {
-        // “\nonscript\mskip-\medmuskip\mkern5mu”
+        // “\nonscript\mskip-\medmuskip\mkern5mu”, where \medmuskip is
+        // 4mu plus 2mu minus 1mu, translates to 1mu space in
+        // display/textstyle and 5mu space in script/scriptscriptstyle.
         if (!options.style.isTight()) {
             inner.push(buildCommon.makeSpan(
-                ["mspace", "negativemediumspace"], [], options));
+                ["mspace", "muspace"], [], options));
+        } else {
+            inner.push(buildCommon.makeSpan(
+                ["mspace", "thickspace"], [], options));
         }
-        inner.push(
-            buildCommon.makeSpan(["mspace", "thickspace"], [], options));
     } else if (options.style.size === Style.DISPLAY.size) {
         inner.push(buildCommon.makeSpan(["mspace", "quad"], [], options));
     } else if (group.value.modType === "mod") {
@@ -40,12 +43,13 @@ const htmlModBuilder = (group, options) => {
             buildCommon.mathsym("d", group.mode)];
         if (group.value.modType === "bmod") {
             inner.push(buildCommon.makeSpan(["mbin"], modInner, options));
-            // “\mkern5mu\nonscript\mskip-\medmuskip”
+            // “\mkern5mu\nonscript\mskip-\medmuskip” as above
-            inner.push(
-                buildCommon.makeSpan(["mspace", "thickspace"], [], options));
             if (!options.style.isTight()) {
                 inner.push(buildCommon.makeSpan(
-                    ["mspace", "negativemediumspace"], [], options));
+                    ["mspace", "muspace"], [], options));
+            } else {
+                inner.push(buildCommon.makeSpan(
+                    ["mspace", "thickspace"], [], options));
             }
         } else {
             Array.prototype.push.apply(inner, modInner);

static/katex.less

@@ -118,6 +118,7 @@
     @ptperem: 10.0;
     @nulldelimiterspace: 1.2em / @ptperem;
 
+    @muspace: 0.055556em;       // 1mu
     @thinspace: 0.16667em;      // 3mu
     @mediumspace: 0.22222em;    // 4mu
     @thickspace: 0.27778em;     // 5mu
@@ -327,6 +328,10 @@
             margin-left: -@thinspace;
         }
 
+        &.muspace {
+            width: @muspace;
+        }
+
         &.thinspace {
             width: @thinspace;
         }