No Main Landmark when Mat Dialog is Open

When running accessibility assessments I consistently face the "Document must have one main landmark" violation when a mat-dialog is open (despite there being a main landmark in the main shell of our app's markup). When dialogs are closed, there is no issue with this specific violation. I've tried adding a role="main" but this actually causes the accessibility engine to detect both main landmarks and thus reports "Can not have more than one main landmark". Adding role="dialog" to the top level div of the dialog container does not resolve the issue either, though it could be possible that this div level is too nested for a11y to detect the role. Could also be possible this is just an issue inherent in modern modal dialog rendering strategies, as I'm seeing the same violation on other sites such as Facebook and Trello. Curious to hear others' thoughts / assessments on this.

*Note* - I get the same results when testing with both axeDevTools and WAVE


按照 issue 中的描述，复现打开的 mat-dialog 场景，并运行 axeDevTools 和 WAVE。确定 missing-main-landmark 违规对于该 dialog 的渲染策略是否属于预期，还是表明组件存在 accessibility 缺陷；完成的标准是得出明确的判断，并在需要时确定受影响的 dialog 入口点。

由索引模型根据 Issue 内容生成。