WebKit recently changed its fixed table layout algorithm to consider the cell's content-box instead of the border-box as was the case in the past. See https://bugs.webkit.org/show_bug.cgi?id=78412 and to some extend http://crbug.com/124816 (it has some reference to this bug too)

If you open http://dev.sencha.com/deploy/ext-2.3...rray-grid.html in any version of Chromium above 19 (that includes the WebKit change), you will see an unneeded horizontal scrollbars.

The work-around is simple: you need to set box-sizing: border-box on your cells' style to keep the existing behavior. The following change solves the issue for me (but don't think this is a definite answer):

.x-grid3-cell {
box-sizing: border-box}