Breaking permissions is advised against, unless... Main rationale for this generic rule of thumb is the complexity that unique permissions brings to permission management in a SharePoint site. However, in addition it can also have a limitating effect on the user experience: persons of which the authorization in a site is limited to only contribute in a specific document library, face issues on upload of documents. Quick investigation revealed that the problem occurs when the document upload is done via drag-and-drop; uploading a document via the explicit 'upload' UI flow does succeed.
On SharePoint platform level, these functional equivalent actions are executed via other flows. In the drag-and-drop scenario, SharePoint executes a CSOM call to read the site properties. But if a person is only authorized to the document library, the 'SelectAllProperties' processQuery action on SharePoint Site level returns 'Access Denied'. Solution is to grant the persons also a limited permission level on the site level, so they are allowed to read the site properties.
With this ‘Read Site Props’ permission level granted at site level, the respective persons are still correct blocked to access entities and content on site level; except for the library to which explicit granted via unique library permissions.
Noteworthy is that the issue does not occur in modern sites, even although also in that context with 'drag-and-drown' execution the site properties are retrieved via CSOM call.
