STRs: - open attached html test case - open inspector - select layout panel, scroll down to grids - click checkbox for the second ".grid" element ER: the checkbox should be checked AR: nothing happens The following exception is logged: > Message: TypeError: fragment.rows.lines[0] is undefined > Stack: > getFirstRowLinePos@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/actors/highlighters/css-grid.js:880:5 > renderFragment@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/actors/highlighters/css-grid.js:916:22 > _update@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/actors/highlighters/css-grid.js:669:7 > update@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/actors/highlighters/auto-refresh.js:240:5 > _startRefreshLoop@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/actors/highlighters/auto-refresh.js:276:5 > show@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/actors/highlighters/auto-refresh.js:110:5 > show@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/actors/highlighters.js:502:12 > handler@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/protocol.js:1082:19 > onPacket@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/main.js:1759:15 > receiveMessage@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/transport/transport.js:761:7

I'll take this one. Simple fix, we just need to check that grid fragments actually contain tracks before drawing them.

Bug 1297072 is almost ready to land and is changing quite a lot the file that I need to fix here. So let's just depend on that bug.

Comment on attachment 8864415 [details] Bug 1358479 - Check if a grid fragment is valid before highlighting it; https://reviewboard.mozilla.org/r/136106/#review139122 Great thanks! ::: commit-message-c4e3b:3 (Diff revision 1) > +Bug 1358479 - Check if a grid fragment is valid before highlighting it; r=jdescottes > + > +Grid fragments returned by the chrome-only el.getGtidFragments() API mega-nit: getGtidFragments -> getGridFragments ::: devtools/server/actors/highlighters/css-grid.js:756 (Diff revision 1) > this.updateCurrentMatrix(); > > // Start drawing the grid fragments. > for (let i = 0; i < this.gridData.length; i++) { > let fragment = this.gridData[i]; > + if (!this.isValidFragment(fragment)) { Maybe move this check inside of renderFragment? Although renderFragment is only called here so it doesn't really matter at the moment.

Pushed by pbrosset@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/73696a2f5ec6 Check if a grid fragment is valid before highlighting it; r=jdescottes

I have successfully reproduced the bug in firefox Nightly 55.0a1 (2017-04-21) (32-bit) with windows 7 (32 bit) Verified as fixed with latest nightly 55.0a1 (2017-05-13) (32-bit) Build ID: 20170513030205 Mozilla/5.0 (Windows NT 10.0; rv:55.0) Gecko/20100101 Firefox/55.0 [testday-20170512]