Fix arrowheads for mhchem <--> (#1451)

* Fix arrowheads for mhchem <-->

* Update screenshots

src/stretchy.js

@@ -153,7 +153,7 @@ const katexImagesData: {
     // In mhchem.sty, min-length is 2.0em. But these arrows might appear in the
     // document as \xrightarrow or \xrightleftharpoons. Those have
     // min-length = 1.75em, so we set min-length on these next three to match.
-    xrightleftarrows: [["baraboveleftarrow", "rightarrowabovebar"], 1.75, 667],
+    xrightleftarrows: [["baraboveleftarrow", "rightarrowabovebar"], 1.75, 901],
     xrightequilibrium: [["baraboveshortleftharpoon",
         "rightharpoonaboveshortbar"], 1.75, 716],
     xleftequilibrium: [["shortbaraboveleftharpoon",

src/svgGeometry.js

@@ -314,21 +314,27 @@ c-1 5-5 9-11 9h-2L532 67 19 159h-2c-5 0-9-4-11-9l-5-22c-1-6 2-12 8-13z`,
     widecheck4: `M1181,340h2l1171,-296c6,0,10,-5,10,-11l-2,-23c-1,-6,-5,-10,
 -11,-10h-1l-1168,273l-1167,-273h-1c-6,0,-10,4,-11,10l-2,23c-1,6,4,11,10,11z`,
 
-    // baraboveleftarrow is from glyph U+21C4 in font KaTeX AMS Regular
-    baraboveleftarrow: `M1 500c30.67-18 59-41.833 85-71.5s45-61.17 57-94.5h23
-c15.33 0 23 .33 23 1 0 .67-5.33 12.67-16 36-16.67 34.67-39 67.33-67 98l-10 11
-h39904v40H96l9 10c27.33 30.67 50.67 65 70 103l14 33c0 .67-7.67 1-23 1h-22
-C116.67 596.33 69 540.67 1 500z M96 480 H400000 v40 H96z
-M1 147 H399905 v40  H1z M0 147 H399905 v40  H0z`,
+    // The next ten paths support reaction arrows from the mhchem package.
 
-    // ditto rightarrowabovebar
-    rightarrowabovebar: `M400000 167c-70.67 42-118 97.67-142 167h-23c-15.33 0
--23-.33-23-1 0-1.33 5.33-13.67 16-37 18-35.33 41.33-69 70-101l7-8h-39905
-v-40h39905c-389 0 0 0 0 0l-7-8c-28.67-32-52-65.67-70-101-10.67-23.33-16-35.67
--16-37 0-.67 7.67-1 23-1h23c11.33 33.33 30 64.833 56 94.5s54.67 53.83 86 72.5z
-M0 147 H399905 v40  H0z M96 480 H400000 v40 H0z M96 480 H400000 v40 H0z`,
+    // Arrows for \ce{<-->} are offset from xAxis by 0.22ex, per mhchem in LaTeX
+    // baraboveleftarrow is mostly from from glyph U+2190 in font KaTeX Main
+    baraboveleftarrow: `M400000 620h-399890l3 -3c68.7 -52.7 113.7 -120 135 -202
+c4 -14.7 6 -23 6 -25c0 -7.3 -7 -11 -21 -11c-8 0 -13.2 0.8 -15.5 2.5
+c-2.3 1.7 -4.2 5.8 -5.5 12.5c-1.3 4.7 -2.7 10.3 -4 17c-12 48.7 -34.8 92 -68.5 130
+s-74.2 66.3 -121.5 85c-10 4 -16 7.7 -18 11c0 8.7 6 14.3 18 17c47.3 18.7 87.8 47
+121.5 85s56.5 81.3 68.5 130c0.7 2 1.3 5 2 9s1.2 6.7 1.5 8c0.3 1.3 1 3.3 2 6
+s2.2 4.5 3.5 5.5c1.3 1 3.3 1.8 6 2.5s6 1 10 1c14 0 21 -3.7 21 -11
+c0 -2 -2 -10.3 -6 -25c-20 -79.3 -65 -146.7 -135 -202l-3 -3h399890z
+M100 241v40h399900v-40z M0 241v40h399900v-40zM0 241v40h399900v-40z`,
 
-    // The next eight paths support reaction arrows from the mhchem package.
+    // rightarrowabovebar is mostly from glyph U+2192, KaTeX Main
+    rightarrowabovebar: `M0 241v40h399891c-47.3 35.3-84 78-110 128-16.7 32
+-27.7 63.7-33 95 0 1.3-.2 2.7-.5 4-.3 1.3-.5 2.3-.5 3 0 7.3 6.7 11 20 11 8 0
+13.2-.8 15.5-2.5 2.3-1.7 4.2-5.5 5.5-11.5 2-13.3 5.7-27 11-41 14.7-44.7 39
+-84.5 73-119.5s73.7-60.2 119-75.5c6-2 9-5.7 9-11s-3-9-9-11c-45.3-15.3-85-40.5
+-119-75.5s-58.3-74.8-73-119.5c-4.7-14-8.3-27.3-11-40-1.3-6.7-3.2-10.8-5.5
+-12.5-2.3-1.7-7.5-2.5-15.5-2.5-14 0-21 3.7-21 11 0 2 2 10.3 6 25 20.7 83.3 67
+151.7 139 205zm96 379h399894v40H0zm0 0h399904v40H0z`,
 
     // The short left harpoon has 0.5em (i.e. 500 units) kern on the left end.
     // Ref from mhchem.sty: \rlap{\raisebox{-.22ex}{$\kern0.5em